Phosphate Concentration and the Putative Sensor Kinase Protein CckA Modulate Cell Lysis and Release of the Rhodobacter capsulatus Gene Transfer Agent

The gene transfer agent ofRhodobacter capsulatus (RcGTA) is a bacteriophage-like genetic element with the sole known function of horizontal gene transfer. Homologues of RcGTA genes are present in many members of the alphaproteobacteria and may serve an important role in microbial evolution. Transcription of RcGTA genes is induced as cultures enter the stationary phase; however, little is known aboutcis -active sequences. In this work, we identify the promoter of the first gene in the RcGTA structural gene cluster. Additionally, gene transduction frequency depends on the growth medium, and the reason for this is not known. We report that millimolar concentrations of phosphate posttranslationally inhibit the lysis-dependent release of RcGTA from cells in both a complex medium and a defined medium. Furthermore, we found that cell lysis requires the genesrcc00555 andrcc00556 , which were expressed and studied inEscherichia coli to determine their predicted functions as an endolysin and holin, respectively. Production of RcGTA is regulated by host systems, including a putative histidine kinase, CckA, and we found that CckA is required for maximal expression ofrcc00555 and for maturation of RcGTA to yield gene transduction-functional particles.