Diversity of Alkane Hydroxylase Systems in the Environment

Diversity of Alkane Hydroxylase Systems in the Environment — The first step in the aerobic degradation of alkanes by bacteria, yeasts, and fungi is catalyzed by oxygenases. These enzymes, which introduce oxygen atoms derived from molecular oxygen into the alkane substrate, play an important role in oil bioremediation and in the cometabolic degradation of compounds such as trichloroethylene and fuel oxygenates. In addition, they are useful biocatalysts and important models for a difficult chemical reaction: the regioand stereospecific activation of C-H bonds. Several unrelated enzyme classes catalyze the oxidation of alkanes. Alkane-degrading yeast strains contain multiple alkane hydroxylases belonging to the P450 superfamily, while many bacteria contain enzymes related to the Pseudomonas putida GPo1 membrane-bound alkane hydroxylase system. Short-chain alkanes are probably oxidized by alkane hydroxylases related to the soluble and particulate methane monooxygenases. Only the membrane-bound enzymes have been studied with respect to their prevalence in environments such as soils or aquifers. Oil & Gas Science and Technology – Rev. IFP, Vol. 58 (2003), No. 4


INTRODUCTION
Saturated hydrocarbons such as linear, branched, and cyclic alkanes are highly reduced forms of carbon that are produced by geochemical processes from decaying plant and algal material.Alkanes constitute about 20-50% of crude oil, depending on the source of the oil.In addition, alkanes (predominantly long-chain compounds) are produced throughout the biosphere by living organisms (plants, algae and bacteria) as a waste product, a structural element, a defense mechanism, or as a chemoattractant.The assimilation of biogenic alkanes is a major process in geochemical terms, but has not been quantified.Recycling of alkanes from natural oil seepage, oil-spills and runoff due to countless dispersed sources has been estimated to amount to several million tons per year [1,2].
The microbial biodegradability of alkanes by microorganisms is well established.Almost a century ago, Söhngen was able to isolate bacteria that degrade methane [3] and longer alkanes [4].Later in the 20th century, alkanes were considered for use as a feedstock for the production of single cell protein.This led to a flood of research in the fifties and sixties, which ended abruptly in response to the oil-crisis in 1973.Alkane-degrading microbes were also investigated by the oil industry for many other purposes, such as petroleum exploration (the presence of alkane degraders was thought to indicate the presence of oil-or gas fields), the treatment of environmental pollution, deparaffinization of jetfuel and diesel oil to reduce viscosity, microbially enhanced oil recovery, and because of corrosion of fuel lines or oil tanks due to bacteria able to grow on oil or petroleum (see for example [5][6][7], and references therein).More recently, alkane hydroxylase genes received increasing attention as markers to predict the potential of different environments for oil degradation (see references in Table 2).An emerging theme is the cometabolic degradation of recalcitrant chemicals such as methyl tert-butyl ether (MTBE) or trichloroethylene (TCE) by bacterial strains growing on short-chain alkanes [8,9].Methane oxidation has often been regarded as a special case of alkane utilization, and was treated separately [10].However, it now appears that at least some of the enzymes that are involved in the degradation of short-chain alkanes are very similar to methane monooxygenases [11].Therefore, some information on these enzymes is included in this review.For a more detailed overview on the molecular biology and biochemistry of methane monooxygenases see Murrell et al. [12].
Alkanes are also known as paraffins (which can be translated as "not enough affinity") as they are chemically quite inert [13].For biodegradation, the alkane molecules have to be activated to allow further metabolic steps.In the absence of oxygen, this is done by the addition of a C 1 (CO 2 ) or a C 4 (fumarate) compound [14].If oxygen is available, the initial activation step is carried out by oxygenases: enzymes that introduce oxygen atoms derived from molecular oxygen in the alkane substrate [15].As the regio-and stereoselective oxidation of nonactivated methyl or methylene-group is a difficult but important chemical reaction, alkane hydroxylases are also studied as model systems for biomimetic catalysts [13].From a biotechnological perspective, alkane hydroxylases are versatile biocatalysts, which carry out a wide range of useful oxidation reactions [16,17].This review summarizes current knowledge on the diversity of alkane hydroxylase systems, their occurence in the environment, and their applications in biocatalysis and bioremediation.

AEROBIC ALKANE DEGRADATION PATHWAYS
Many microbial genera include strains that are able to grow on alkanes.Table 1 lists a small selection of genera that include strains from which genes involved in alkane degradation were cloned, or genera that are often mentioned in alkane degradation studies.Alkanes are usually activated by terminal oxidation to the corresponding primary alcohol, which is further oxidized by alcohol and aldehyde dehydrogenases.The resulting fatty acids enter the β-oxidation cycle (for general reviews see [6,10,18]).Shortchain alkanes are metabolized via terminal as well as subterminal oxidation [19], and many methanotrophs cooxidize short-chain alkanes at terminal as well as subterminal positions [7,20].Subterminal oxidation has also been detected for longer alkanes, e.g. in Penicillium, Bacillus, Pseudomonas [10], and more recently in Rhodococcus sp.Q15 [21].The secondary alcohols are converted to the corresponding ketone [22], which is oxidized by a Baeyer-Villiger monooxygenase to an ester [23][24][25].The ester is subsequently hydrolyzed by an esterase to an alcohol and a fatty acid [19,26].In some cases, both ends of the alkane substrate are oxidized, which has been exploited for the production of dicarboxylic acids by yeasts as well as bacteria [27,28] (Fig. 1).

Sphingomonas Streptomyces
Information mainly from [7,18].A (+) indicates that information on the genetics of alkane degradation is available for strains belonging to the genus.
the electrons to the alkane hydroxylase.Recently, enzyme systems related to the P. putida GPo1 alkane hydroxylase were cloned from bacterial strains belonging to several different genera [34,50,[52][53][54].While the P. putida GPo1 system acts on alkanes ranging from pentane to dodecane (C 5 -C 12 ) [42,55], most related membrane-bound alkane hydroxylases solely hydroxylate alkanes containing more than 10 carbon atoms [34] (see below).Several cytochrome P450 isozymes involved in alkane assimilation were recently cloned from the yeasts Candida maltosa, Yarrowia lipolytica and Debaryomyces hansenii [29,56,57].They belong to the CYP52 gene family of the microsomal class II P450s, and receive electrons from NADPH (nicotinamide adenine dinucleotide phosphate) via FAD-and FMN-containing reductases.The CYP52 enzymes were found to have slightly different substrate specificities: some act primarily on C 12 or C 16 alkanes, while other enzymes also oxidize fatty acids at the ω-position.The CYP52 gene family may play a significant role in the degradation of alkanes in oil-contaminated environments as yeast and fungi have been shown to overgrow bacteria in sandy soil contaminated with C 10 -C 15 n-alkanes [58].

Alkane
Pathways for the degradation of alkanes by terminal, sub-and biterminal oxidation.Terminal oxidation leads to the formation of fatty acids, which enter the β-oxidation pathway.Alternatively, ω-hydroxylation by a fatty acid monooxygenase or alkane hydroxylase may take place, leading to dicarboxylic acids.Subterminal oxidation gives rise to secondary alcohols, which are oxidized to ketones.A Baeyer-Villiger monooxygenase converts ketones to esters, which are subsequently cleaved by an esterase.
Reports on bacterial cytochrome P450 systems involved in alkane degradation are quite scarce.Cardini and Jurtshuk described a Corynebacterium sp.7E1C (in recent literature known as Rhodococcus rhodochrous) that contains an octane hydroxylating cytochrome P450 enzyme [59].Another cytochrome P450 enzyme involved in alkane hydroxylation was isolated [60] and recently cloned from Acinetobacter calcoaceticus EB104 [30].As this P450 showed less than 40% sequence identity to other sequences in the P450 superfamily it was assigned to a new bacterial class I P450s family named CYP153.Electron transfer to the bacterial class I enzymes is mediated by ferredoxin reductase, an FAD-containing protein which accepts electrons from NAD(P)H, and ferredoxin, an iron-sulfur protein which transfers the electrons to the P450 monooxygenase [61].
Butane-utilizing strains were shown to contain several unrelated alkane hydroxylases, based on responses to ethylene (which inactivates cytochrome P450 enzymes by alkylating the heme group), acetylene (a mechanism-based inactivator of soluble methane monooxygenases (sMMO), membrane-bound methane monooxygenases (pMMO) and ammonia monooxygenase (AMO)) and allyl thiourea (which chelates copper, and reversibly inhibits pMMO and AMO) [11].One of these strains, Nocardioides sp.CF8, was shown to contain a membrane-bound alkane hydroxylase related to the P. putida GPo1 AlkB, and a copper-dependent light sensitive enzyme similar to pMMO [32].A butane monooxygenase recently cloned from Pseudomonas butanovora is quite similar to sMMO [12,33]; its protein A α and β subunits, and protein B show more than 60, 50, and 40% amino acid sequence identity, respectively, to the corresponding subunits of sMMOs.
A gene which allows isolates of a novel Pseudomonas sp.named P. indica IMT37 [36] to grow on butane had no detectable homology or similarity with other alkane hydroxylase systems [37].Here, further biochemical work is necessary to elucidate the role of the encoded peptide in butane oxidation.The same holds for a new type of intermolecular dioxygenase, which appears to oxidize two molecules of propane with one molecule of molecular oxygen [38].A copper-dependent alkane dioxygenase was reported for Acinetobacter sp.M-1 [35], a strain that was later shown to contain two membrane-bound alkane hydroxylases related to P. putida GPo1 AlkB as well [54].

DIVERSITY OF MEMBRANE-BOUND ALKANE HYDROXYLASE SYSTEMS
Biochemical studies and molecular genetic analysis of the P. putida GPo1 alkane hydroxylase system have led to a detailed understanding of C 5 -C 12 n-alkane metabolism by this strain [40,41], while other alkane-degraders (except for the methanotrophs) were studied in much less detail.For example, even though hydrocarbon utilization is very common amongst strains belonging to the Corynebacterium-Mycobacterium-Nocardia (CMN) complex, until recently only a few CMN strains, such as the R. rhodochrous strains 430 Structure of membrane-bound alkane hydroxylase systems.The membrane-bound alkane hydroxylases possess 6 transmembrane helices [44], and 4 conserved histidine-rich motifs (H) [50,51].The 2 iron atoms in the active site are marked (•).Rubredoxin contains an iron atom marked (•) that is liganded by 4 cysteines.Several strains contain more than 1 rubredoxin, or larger proteins consisting of N-terminal and C-terminal rubredoxin domains connected by a linker [47].Rubredoxin reductase is an FAD-containing NADHdependent reductase [49].
In 1998, a membrane-bound alkane hydroxylase that was quite distantly related to the P. putida GPo1 AlkB was cloned from Acinetobacter sp.ADP1, and shown to act on C 12 and longer n-alkanes [52].The genome sequence of Mycobacterium tuberculosis H37Rv [68] was also found to encode an AlkB-homolog, which was later shown to be functional by heterologous expression [34].Comparison of the three alkane hydroxylase sequences showed that four histidine-containing motifs were well conserved, three of which were also present in the distantly related membranebound desaturases [51].Based on two of the motifs, highly degenerate primers were developed that amplified internal gene fragments of alkB homologs from gram-negative as well as gram-positive strains [32,50,53,69] (Table 3).In the initial PCR study, most strains that were able to grow on alkanes yielded alkB gene fragments [50], and the same was found for a set of Rhodococcus strains isolated from soil in Bremen, Germany [86,87].However, in a collection of 17 hexane-degrading strains isolated from a trickling bed bioreactor that removed hexane from an airstream [88], only 5 strains possessed an alkB-homolog [86].This suggests that many strains able to degrade shorter alkanes possess enzyme systems that are unrelated to the membrane-bound alkane hydroxylase of P. putida GPo1.On the other hand, many strains (especially Rhodococcus and P. aeruginosa strains) possess multiple-quite divergent-alkane hydroxylases [67,86,89] (Fig. 3, Table 3).
The alkB homologs amplified from the different strains showed high sequence divergence: the branches of a distance tree based on the amino-acid translations of the DNA sequences obtained by the PCR approach, molecular cloning, and genome sequencing had sometimes only 40% sequence identity (Fig. 3).To demonstrate that the alkB gene homologs indeed encode functional enzymes, special expression vectors [91] and host strains for functional heterologous expression of alkane hydroxylases [34] were constructed.The host strains (Pseudomonas fluorescens KOB2∆1, an alkB knockout derivative of P. fluorescens CHA0, and P. putida GPo12(pGEc47∆B), a derivative of P. putida GPo1 lacking the alkB gene) supply the electron transfer components rubredoxin and rubredoxin reductase, the enzymes that carry out further metabolic steps leading to fatty acids, and, in the case of P. fluorescens KOB2∆1, also an uptake system for alkanes longer than C 12 .The host-strains made functional expression of alkane hydroxylases from strains as diverse as Mycobacterium tuberculosis, Prauserella rugosa, Rhodococcus erythropolis, Burkholderia cepacia, Pseudomonas aeruginosa, and Alcanivorax borkumensis possible [34,53,89] (marked by "+" in Fig. 3).However, they only allow functional analysis of enzymes that oxidize n-alkanes ranging from C 5 -C 16 ; alkane hydroxylases which oxidize longer alkanes could not be investigated yet.Additional membranebound alkane hydroxylases were cloned from Acinetobacter sp.strain M-1 and A. calcoaceticus EB104, and shown to be functional by expression in an alkM knockout strain of Acinetobacter sp.ADP1 [50,54].Several genome sequences (e. g.Burkholderia mallei, P. aeruginosa, Legionella pneumophilia, M. tuberculosis, Mycobacterium bovis) that have recently become available also contain alkB homologs (marked by "*" in Fig. 3).Most of these bacteria are  Unrooted phylogenetic tree based on an alignment of partial amino acid sequences of (putative) membrane-bound alkane hydroxylases.The distance tree was generated by ClustalX [90] from a manual alignment of the (putative) alkane hydroxylases.Only the segment corresponding to the 550 basepair fragments obtained in PCR experiments with highly degenerate primers based on histidine box 1 and histidine box 4 were used for the alignment.Alkane hydroxylases that were shown to be functional by heterologous expression or gene knockouts are indicated by (+), while sequences from unfinished genome sequences are indicated by (*).Scale bar, 0.1 substitution per amino-acid site.
facultative pathogens that may utilize alkanes as a carbonsource in environments such as soil or water.
A careful inspection of the distance tree shows that there is no clear linkage between the diversity of the alkB genes and phylogenetic lines (Fig. 3).For example, AlkB homologs from fluorescent pseudomonads are almost as divergent as the entire collection, while the four AlkB-homologs in R. erythropolis NRRL B-16531 and Q15 are as divergent as the whole set of sequences from gram-positive strains [89].Some strains contain multiple AlkB sequences that are on completely different branches-A.borkumensis, and several P. aeruginosa strains that are able to grow on n-octane as well as the longer n-alkanes (Smits et al., submitted; van Beilen, unpublished data).On the other hand, the Burkholderia and P. fluorescens sequences are so closely related to some sequences from gram-positive strains that cross-reactions in Southern and dot-blots are possible (>70% DNA sequence identity).In part, the failing link between phylogenetic lineage and AlkB-diversity is clearly due to horizontal gene transfer.For example, the P. putida GPo1 alk-genes are located on a large broad host-range plasmid named OCT [92], while the closely related P. putida P1 alkgenes are located on a class I transposon [41].
Our present knowledge on the structure-function relationship of the class of integral membrane alkane hydroxylases is limited to the folding topology [44] and the likely involvement of conserved histidines in binding of the 2 active-site iron atoms [51].The AlkB sequences show significant sequence divergence, but the membrane folding of these alkane hydroxylases appears to follow the same pattern: the 6 hydrophobic stretches that are likely to span the cytoplasmic membrane [51] (Fig. 2) are highly conserved in all sequences, while the same is true for the conserved histidines.It is not yet possible to identify residues involved in other aspects of alkane hydroxylase function.Residues that are involved in binding of rubredoxin are likely to be conserved between all alkane hydroxylases, as rubredoxins can be exchanged between alkane hydroxylase systems from all strains tested [47].However, these residues cannot be distinguished from other conserved residues.Similarly, the three medium chain-length alkanes hydroxylases from P. putida GPo1, P. putida P1, and A. borkumensis AP1 are too closely related to each other to distinguish residues conserved by chance from residues conserved because of functional constraints.
The organization of genes involved in alkane oxidation varies strongly among the different alkane degrading bacteria (Fig. 4).In most strains, genes involved in alkane degradation seem to be distributed over the genome.Except in the case of R. erythropolis [89], none of the rubredoxin reductase genes is located close to an alkane hydroxylase, perhaps because they are also involved in other pathways, and require a different type of regulation.In contrast, most rubredoxin genes are located immediately downstream of the alkane hydroxylase genes.Those that are located elsewhere are encoded directly upstream of rubredoxin reductase genes.Interestingly, the alk-gene cluster around alkB1 of A. borkumensis (a recently discovered seawater bacterium that only grows on alkanes [73]) is very similar to the alk gene clusters of P. putida strains GPo1 and P1, with respect to gene organization as well as sequence.The P. putida alk genes have a significantly lower G+C content than the rest of the genome, and are encoded on a putative catabolic transposon [41].These comparisons suggest that the A. borkumensis alk genes may have ended up in this strain by horizontal gene transfer as well.Interestingly, the alkane hydroxylase genes are accompanied by alcohol and aldehyde dehydrogenase genes only in these strains (Fig. 4).

DETECTION OF ALKANE HYDROXYLASE GENES IN THE ENVIRONMENT
Most of the enzyme systems described above have been discovered recently, and evidence that similar systems are present in other bacteria is still scarce.A gene probe based on the IMT37 sequence essential for butane oxidation detected 435 Fuel oil-contaminated site Dot-blots DNA extracted from soil shows no significant hybridization with an alkB-gene probe [100] Shallow aquifer Southern blots 10-20% of the total bacterial community hybridizes with alkB gene probe with an alkB gene probe [101] Various sources PCR with highly degenerate Most alkane-degrading strains contain distantly related alkB homologs (54 bacterial strains) primers for alkB homologs (homology not sufficient for Southern or dot-blots) [50] Shallow aquifer Dot-blots with DNA extracted alkB genotypes start at 11% of total community, and peak at (natural attenuation site) from aquifer samples 52% after injection of synthetic jet fuel in the aquifer [102] Various sources Southern, colony and dot-blots, alkB genes (close homologs) are widespread only in short-chain n-alkane (54 bacterial strains) PCR followed by Southern degrading pseudomonads [76] Rhizosphere vs. bulk soil Multiplex PCR At a contaminated site alkB was 10 times more prevalent in the endophytic community compared to the bulk soil community [103] Propane and butane-Dot-blots 8 of 15 strains (including pseudomonads and rhodococci) utilizing bacteria gave a strong signal, and 7 a weak signal with the IMT37 gene [37] Land treatment unit (LTU) PCR, terminal restriction alkB increased in abundance during the first 3 weeks of LTU operation, fragment length polymorphism and comprised > 80% of the total PCR products [104] Arctic and Antarctic soil PCR-hybridization Rhodococcal alkB genes occur in contaminated and pristine soils, and colony hybridization P. putida alkB occurs more frequently in contaminated soils [105] similar genes in other butane degraders (pseudomonads, rhodococci and unidentified bacteria).In a dot-blot experiment, 8 out of 15 propane or butane-degrading bacteria gave a strong signal with the gene-probe, while the remaining 7 gave a weak but detectable signal [37].
The prevalence in the environment of the CYP153 P450 alkane hydroxylase family, the first member of which was recently cloned from Acinetobacter sp.EB104 [30], and the yeast CYP52 P450 family, has not been investigated yet.However, hexane or octane inducible cytochrome P450s were detected in several Rhodococcus, Bacillus megaterium and Candida apicola strains [59,93].Indications that the butane monooxygenases studied by Arp and coworkers [11,32,33] are also widespread come from studies on the cometabolic degradation of TCE and other chlorinated compounds by bacteria growing on short-chain alkanes [9,11].Soluble and particular MMOs are known to oxidize the same compounds [20], and are ubiquitous in many environments (see for example [94][95][96]).This also implies that some of the gene diversity detected with primers that amplify pMMO and sMMO genes may well be due to shortchain alkane-degrading bacteria instead of methanotrophs.
Because the sequence of the P. putida GPo1 alkB gene was already published in 1989 [43], several research groups have used this gene as a probe in ecological studies, with conflicting results (Table 4).While in some studies alkB homologs were detected in 10-40% of the bacterial population [97,101,102], other groups did not detect alkB homologs at all [99,100].AlkB homologs that are closely related to the P. putida GPo1 enzyme appear to be common in gram-negative strains only (probably pseudomonads) [76].This is supported by the observation that genes almost identical to the P. putida GPo1 alkB gene were found in several P. putida, P. aeruginosa, and P. mendocina strains [67,87], but not in other strains.An exception is the alkB1 gene found in A. borkumensis AP1, which is sufficiently closely related to the P. putida alkB genes for cross-reactions in Southern blots.This supports the notion that horizontal gene transfer has played a role in spreading this genotype [34].Rhodococcal alkB homologs were only cloned recently [50,86,89], but first reports indicate that these genotypes are widespread in Arctic and Antarctic soils, and in other environments [86,105].

APPLICATIONS OF ALKANE HYDROXYLASE SYSTEMS
A strong incentive for research on alkane hydroxylases was the early observation that the P. putida alkane hydroxylase oxidizes alkenes to epoxides [106], while P. aeruginosa KSLA 473 (which contains an enzyme system that is identical to the P. putida GPo1 alkane hydroxylase [67]) hydroxylates a wide range of branched and cyclic alkanes, and alkylbenzenes [107].P. putida GPo1 was found to oxidize many other substrates as well (including ethylsubstituted heterocyclic compounds, allyl phenyl or allyl benzyl ethers, thioethers, branched alkanes, etc.) with excellent regio-and/or stereoselectivity [16,55,[108][109][110][111].Screening of a collection of alkane-degrading strains for the regio-and stereoselective hydroxylation of N-benzylpyrrolidine showed that 25-30% of the strains oxidized this substrate to N-benzyl-3-hydroxypyrrolidine with varying enantiomeric excess [110].The best strain in this study was a hexane-degrading Sphingomonas sp.strain HXN200, isolated from the trickling-bed filter mentioned earlier [88].
In subsequent work, Sphingomonas sp.strain HXN200 was shown to hydroxylate many 4-, 5,-and 6-ring alicyclic compounds with rates of up to 10-20 µmol min -1 g -1 cell dry weight [17,112,113].The stereoselective oxidation of isopropyl moieties by alkane-grown Rhodococcus and Pseudomonas strains has also been studied in some detail [85,114,115].
A new application area of alkane hydroxylases is the cometabolic degradation of TCE, MTBE, and related compounds by bacteria able to grow on short-chain alkanes (typically propane to pentane) [8,[116][117][118][119][120].The propane and TCE degrading strain P. butanovora was shown to contain butane monooxygenase genes that were closely related to sMMO [33].

CONCLUSIONS AND OUTLOOK
Alkanes are ubiquitous in the environment due to biogenic production and oil pollution, and many aerobic microorganisms are able to use these highly reduced hydrocarbons as a sole carbon-and energy source.The oxygenases that are required for the initial activation of alkanes belong to several different enzyme classes, some of which only oxidize shortchain alkanes, while others oxidize medium and long-chain alkanes.Membrane-bound alkane hydroxylases related to AlkB of P. putida GPo1 have been found in many bacteria able to grow on C 5 -C 16 alkanes, and have also been detected in environments such as soils and aquifers.Other enzyme systems involved in alkane assimilation have been studied only in a limited number of strains (often just 1 or 2).Future research efforts are likely to yield additional and perhaps completely novel alkane hydroxylase systems.A major challenge will be to explain the parallel evolution of such diverse families of alkane hydroxylases with essentially the same function.For this purpose, the roles of the individual enzyme systems in their hosts and ecological niches need to be assessed in detail.Important elements in the required physiological studies will be the quantification of kinetic parameters such as the k cat and K m of various enzyme systems for a variety of alkanes and oxygen, according to standardized protocols.Potential applications in bioremediation and biocatalysis are a strong incentive to study cometabolic oxidations by alkane hydroxylase systems in much greater detail.Studies of alkane hydroxylase gene diversity, coupled with information on substrate range, induction, enzyme kinetics and host properties, should help to optimize the biodegradative activity of indigenous hydrocarbon degrading strains, benefit biocatalytic applications, and promote fundamental research on the activation of oxygen by enzymes and biomimetic catalysts.

Figure 4
Figure 4 Organization of alk genes in different microorganisms.The function of the encoded proteins is indicated below.If the ORFs are linked by a line, the sequence is contiguous.If the ORFs are not linked by a line, the genes are located elsewhere on the chromosome.The triangle indicates the direction of transcription.

TABLE 2
Enzyme classes involved in the oxidation of alkanes

TABLE 4
Studies on the occurrence, abundance, and/or diversity of alkB-homologues in different environments or strain collections