ABSTRACT
The conjugative transfer of the Streptococcus faecalis plasmid pAD1 is characterized by a 10,000-fold increase in frequency following sex pheromone (cAD1) induction. Before the increase in plasmid transfer, donor cells synthesize a proteinaceous adhesin that facilitates the formation of mating aggregates. Four novel surface proteins appearing after exposure of pAD1-containing cells to sex pheromone have been identified. Thirty minutes after induction, a 130-kilodalton (kDa) protein was detectable by Western blotting. A 74-kDa protein, the major species present, and a pair of bands at 153 and 157 kDa were evident 45 min after induction. Induced cells containing another conjugative S. faecalis plasmid, pPD1, gave rise to three high-molecular-weight proteins of the same size (130, 153, and 157 kDa) as those synthesized by pAD1-containing cells. These proteins cross-reacted with antisera raised against induced cells containing pAD1. However, the major protein species produced by pPD1-containing cells had a molecular weight of 78,000 and did not cross-react significantly with the corresponding band of the pAD1 system. Pheromone-induced transfer of the two plasmids, when both were present in the same cell, was independent; induction was limited to the pheromone-specified plasmid. The possibility that lipoteichoic acid might act as a receptor (binding substance) for the induced adhesin protein was also explored. Free lipoteichoic acid (isolated from S. faecalis) inhibited clumping of induced cells, apparently by acting as a competitive inhibitor of the cellular binding substance.
