The expression of a novel antisense gene mediates incompatibility within the large repABC family of alpha-proteobacterial plasmids.
Large extrachromosomal replicons in many members of the alpha-proteobacteria encode genes that are required for plant or animal pathogenesis or symbiosis. Most of these replicons encode repABC genes that control their replication and faithful segregation during cell division. In addition to its chromosome, the plant endosymbiont Sinorhizobium meliloti also maintains the 1.4 Mb pSymA and 1.7 Mb pSymB symbiotic megaplasmids both of which are repABC-type replicons. In all repABC loci that have been characterized, an apparently untranslated intergenic region between the repB and repC genes encodes a strong incompatibility determinant (referred to as incalpha). Here we report the isolation of mutations within the incalpha regions of pSymA and pSymB that eliminate incompatibility. These mutations map to and inactivate a promoter in the intergenic region that drives the expression of an approximately 56 nucleotide untranslated RNA molecule that mediates incompatibility. This gene, that we have named incA, is transcribed antisense to the repABC genes. Our analysis suggests that the incA gene is conserved in repABC loci from a diverse spectrum of bacteria.
MacLellan
Shawn R
SR
Department of Biology, Life Sciences Building, McMaster University, 1020 Main St. West, Hamilton, Ontario, Canada, L8S 4K1.
Smallbone
Laura A
LA
Sibley
Christopher D
CD
Finan
Turlough M
TM
D016960:Agrobacterium tumefaciens / Q000235:genetics; D020561:Alphaproteobacteria / Q000235:genetics*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D004265:DNA Helicases / Q000235:genetics; D021901:DNA, Intergenic / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D008969:Molecular Sequence Data; D009154:Mutation; D010957:Plasmids / Q000235:genetics*; D011401:Promoter Regions, Genetic; D016372:RNA, Antisense / Q000235:genetics / Q000378:metabolism*; D012093:Replicon / Q000235:genetics*; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000378:metabolism; D013559:Symbiosis; D015534:Trans-Activators / Q000235:genetics
D001426:Bacterial Proteins; D021901:DNA, Intergenic; D004268:DNA-Binding Proteins; D016372:RNA, Antisense; C048817:RepB protein, Bacteria; D015534:Trans-Activators; C045264:replication initiator protein; D004265:DNA Helicases