Analysis of the genome of Geobacter sulfurreducens revealed four genes encoding putative symporters with homology to ActP, an acetate transporter in Escherichia coli. Three of these genes, aplA, aplB and aplC, are highly similar (over 90 % identical) and fell within a tight phylogenetic cluster (Group I) consisting entirely of Geobacter homologues. Transcript levels for all three genes increased in response to acetate limitation. The fourth gene, aplD, is phylogenetically distinct (Group II) and its expression was not influenced by acetate availability. Deletion of any one of the three genes in Group I did not significantly affect acetate-dependent growth, suggesting functional redundancy. Attempts to recover mutants in which various combinations of two of these genes were deleted were unsuccessful, suggesting that at least two of these three transporter genes are required to support growth. Closely related Group I apl genes were found in the genomes of other Geobacter species whose genome sequences are available. Furthermore, related genes could be detected in genomic DNA extracted from a subsurface environment undergoing in situ uranium bioremediation. The transporter genes recovered from the subsurface were most closely related to Group I apl genes found in the genomes of cultured Geobacter species that were isolated from contaminated subsurface environments. The increased expression of these genes in response to acetate limitation, their high degree of conservation among Geobacter species and the ease with which they can be detected in environmental samples suggest that Group I apl genes of the Geobacteraceae may be suitable biomarkers for acetate limitation. Monitoring the expression of these genes could aid in the design of strategies for acetate-mediated in situ bioremediation of uranium-contaminated groundwater.
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