Discovered by genomics: putative reductive dehalogenases with N-terminus transmembrane helixes

Attempts for bioremediation of toxic organohalogens resulted in the identification of organohalide-respiring bacteria harbouring reductive dehalogenases (RDases) enzymes. RDases consist of the catalytic subunit (RdhA, encoded by rdhA) that does not have membrane-integral domains, and a small putative membrane anchor (RdhB, encoded by rdhB) that (presumably) locates the A subunit to the outside of the cytoplasmic membrane. Recent genomic studies identified a putative rdh gene in an uncultured deltaproteobacterial genome that was not accompanied by an rdhB gene, but contained transmembrane helixes in N-terminus. Therefore, rather than having a separate membrane anchor protein, this putative RDase is likely a hybrid of RdhA and RdhB, and directly connected to the membrane with transmembrane helixes. However, functionality of the hybrid putative RDase remains unknown. Further analysis showed that the hybrid putative rdh genes are present in the genomes of pure cultures and uncultured members of Bacteriodetes and Deltaproteobacteria, but also in the genomes of the candidate divisions. The encoded hybrid putative RDases have cytoplasmic or exoplasmic C-terminus localization, and cluster phylogenetically separately from the existing RDase groups. With increasing availability of (meta)genomes, more diverse and likely novel rdh genes are expected, but questions regarding their functionality and ecological roles remain open.