Downstream regulation of int gene expression by the b2 region in phage lambda

Expression of the int gene after phage λ infection normally requires the products of genes cII and cIII. However, when the phage carries a deletion in the nonessential b2 region adjacent to int, efficient synthesis of active Int protein does not require cII and cIII function. This inhibition of Int synthesis by nucleotide sequences downstream from the int structural gene behaves in a cis-dominant fashion in mixed infections. It is specific for P_L- and not p_I-initiated transcripts. Based on these observations, and those of others, a model is proposed in which Int translation from the p_L transcript is inhibited by the interaction of downstream b2 nucleotide sequences and nucleotide sequences in the int region. The data imply a novel temporal mechanism regulating prophage λ induction: circularization of the prophage genome results in the transposition of inhibitory b2 region sequences next to int and blocks further Int protein synthesis beyond the low level required for excision. As a consequence of this process, the control of int expression is transferred from the p_L promoter to p_I and the cII/cIII system. Such a genetic regulatory mechanism involving the rearrangement of genetic elements downstream from a structural gene may be of general use during development in other systems.