Sex-pheromone-plasmid-bearing strains of Enterococcus faecalis react with sex pheromone to induce the expression of an adhesin, the so-called aggregation substance, on their cell surface. Here we show that, by complementation studies, for sex-pheromone plasmid pAD1, expression of the structural gene asa1, coding for an aggregation substance, is mediated by a diffusible factor encoded on pAD1. We were able to demonstrate that a small open reading frame, traE1, is sufficient for transcription of the operon containing asa1. A model for expression of asa1 under the influence of the positive regulator is presented, which is supported by our observation that regulation involves an all-or-nothing induction phenomenon, leading to cells either fully expressing asa1 or not at all.