Book Description

Vinyl chloride (VC) is a widespread groundwater pollutant and Group 1 carcinogen. Microbial respiration of VC is both critical for complete remediation of chloroethenes in situ, and a unique physiology only observed by certain strains of Dehalococcoides. Two different genes independently encoding VC respiration in Dehalococcoides, vcrA and bvcA, were identified previously, each a member of the diverse family of reductive dehalogenase homologous genes, or rdhA. In this thesis I report that vcrA and bvcA are among a subset of putative 'foreign' rdhA with a low GC3 codon usage that favors the nucleotide T, even though tRNAs recognizing T-ending codons are categorically absent in Dehalococcoides genomes. Comparative genomics of the first two complete genome sequences of microorganisms able to respire VC, Dehalococcoides strains VS and BAV1, reveals that vcrA and bvcA are both located within recently integrated, but different, genomic islands (GIs). These islands have different predicted integration sites and different relative positions in the genome, suggesting that bvcA and vcrA were acquired independently and through different mechanisms. In particular, the vcrA-containing GI appears to have integrated at the single-copy tmRNA encoding gene, ssrA, along with many other homologous elements that site-specifically integrate/excise at ssrA -- some of which contain other rdhA. A detailed analysis of these 'ssrA-GIs' in Dehalococcoides identifies the precise position of insertion as well as a conserved module of syntenic integration-associated genes that includes the likely ssrA-specific integrase. Further analysis of (meta)genomic data, as well as targeted sequencing from 4 additional VC-respiring cultures, provided a total of 8 syntenic vcr-GIs from independently derived cultures. Evolutionary estimates for the age of divergence of these 8 vcrA sequences is not confidently distinguishable from the first industrial synthesis of chlorinated ethenes approximately 100 years ago. By contrast, the estimated age of divergence of Dehalococcoides strains far precedes industrial civilization. Overall, ssrA-GIs appear to be a major contributor to the second of two high plasticity regions, genetically dynamic regions that interrupt an otherwise stable, syntenic, and streamlined Dehalococcoides genome -- among the smallest of any free living microorganism. The apparent compartmentalization of Dehalococcoides genome dynamics within specialized regions may enhance opportunistic adaptation to (new) organohalide respiratory niches while protecting a core genome that is highly adapted to life in the anoxic subsurface, exemplified here by the recent site-specific acquisition of VC reductase genes.