Cooperative Interaction of CI Protein Regulates Lysogeny of Lactobacillus casei by Bacteriophage A2

Abstract

The temperate bacteriophage A2 forms stable lysogens in Lactobacillus casei . The A2-encoded cI product (CI), which is responsible for maintaining the A2 prophage in the lysogenic state, has been purified. The CI protein, which is a monomer of 25.3 kDa in solution, specifically binds to a 153-bp DNA fragment that contains two divergent promoters, P L and P R . These promoters mediate transcription from cI and a putative cro , respectively. Three similar, although not identical, 20-bp inverted repeated DNA segments (operator sites O 1 , O 2 , and O 3 ) were found in this segment. CI selectively interacts with O 1 , which is placed downstream from the transcription start point of the cro gene, and with O 2 and O 3 , which overlap with the −35 region of the two promoters. Using a heterologous RNA polymerase, we have determined the transcription start points of P L and P R . CI exerts a negative effect on the in vitro transcription of P R by repositioning the RNA polymerase in a concentration-dependent manner. CI, when bound to O 1 and O 2 , enhances the positioning of the RNA polymerase with the P L promoter. Our data indicate that the CI protein regulates the lytic and lysogenic pathways of the A2 phage.