A QUORUM-SENSING SYSTEM IN THE FREE-LIVING PHOTOSYNTHETIC BACTERIUM RHODOBACTER-SPHAEROIDES

Rhodobacter sphaeroides is a free-living, photoheterotrophic bacterium known for its genomic and metabolic complexity. We have discovered th at this purple photosynthetic organism possesses a quorum-sensing syst em. Quorum sensing occurs in a number of eukaryotic host-associated gr am-negative bacteria. In these bacteria there are two genes required f or quorum sensing, the luxR and luxI homologs, and there is an acylhom oserine lactone signal molecule synthesized by the product of the luxI homolog. In R. sphaeroides, synthesis of a novel homoserine lactone s ignal, 7,8-cis-N-(tetradecenoyl)homoserine lactone, is directed by a l uxI homolog termed cerI. Two open reading frames immediately upstream of cerI are proposed to be components of the quorum-sensing system. Th e first of these is a luxR homolog termed cerR, and the second is a sm all open reading frame of 159 bp. Inactivation of cerI in R. sphaeroid es results in mucoid colony formation on agar and formation of large a ggregates of cells in liquid cultures. Clumping of CerI mutants in liq uid culture is reversible upon addition of the acylhomoserine lactone signal and represents a phenotype unlike those controlled by quorum se nsing in other bacteria.