PHASE VARIATION OF ENTEROCOCCUS-FAECALIS PAD1 CONJUGATION FUNCTIONS RELATES TO CHANGES IN ITERON SEQUENCE REGION

pAD1 (60 kb) is a conjugative, hemolysin/bacteriocin plasmid in Entero coccus faecalis. It confers a mating response to the peptide sex phero mone cAD1 produced by recipient (plasmid-free) cells, leading to highl y efficient plasmid transfer in broth matings, Control of the physiolo gical response to cAD1 can been overridden by a reversible phase varia tion event at frequencies on the order of 10(-4) to 10(-3) per cell pe r generation (L. T, Pontius and D. B. Clewell, Plasmid 26:172-185, 199 1), The variant forms are designated Dry(c) and Dry(+), which reflects the colony morphologies of cells whose conjugation functions are swit ched on and off, respectively. Here we show that Dry(c) variants exhib it a structural change in a region between repA and repB that contains two clusters of 8-bp iterons. The change involved a 31- or 32-bp incr ease in size of this region. In three of four independent variants exa mined, one of the iteron clusters increased in size from 13 to 17 iter ons. When iteron DNA was placed on a multicopy plasmid and introduced into a wild-type pAD1 derivative, the Dry(c) phenotype was generated, Since traA, a key negative regulator of conjugation, bears several cen trally located iteron-like sequences with the same orientation, we spe culate that the protein(s) that normally binds iterons (possibly RepA and/or RepB) blocks traA transcription in Dry(c) variants.