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Summary

The gene transfer agent produced by Rhodobacter capsulatus (RcGTA) resembles a small tailed bacteriophage that packages almost random genomic DNA segments that may be transferred to other R. capsulatus cells. Gene transfer agents are produced by a number of prokaryotes; however, no receptors have been identified. We investigated the RcGTA recipient capability of wild-type R. capsulatus cells at different culture growth phases, and found that the frequency of RcGTA-dependent acquisition of an allele increases as cultures enter the stationary phase. We also found that RcGTA adsorption to cells follows a similar trend. RcGTA recipient capability and adsorption were found to be dependent on the GtaR/I quorum-sensing (QS) system. Production of an extracellular polysaccharide was found to be regulated by GtaR/I QS, as was production of the cell capsule. A number of QS-regulated putative polysaccharide biosynthesis genes were identified, and mutagenesis of two of these genes, rcc01081 and rcc01932, yielded strains that lack a capsule. Furthermore, these mutants were impaired in RcGTA recipient capability and adsorption, as was a non-encapsulated wild-type isolate of R. capsulatus. Overall, our results indicate that capsular polysaccharide is a receptor for the gene transfer agent of R. capsulatus, RcGTA.