Retroregulation: control of integrase expression by the b2 region of bacteriophages lambda and 434

Genetic fusions constructed by a combination of in vivo and in vitro techniques have been used to investigate the cis-regulatory role of a 250-base-pair segment of b2 DNA in the expression of int transcripts originating from three different promoters PI, PL, and PTRP. It has been established that (a) the PI and PTRP transcripts, which produce functional int protein, are efficiently terminated by the b2 segment, (b) in the same test system, the PL transcript, which does not result in functional integrase, is not terminated by the b2 segment, (c) this b2 (sib) effect does not depend on genes lying between int and N on the phage genome, (d) the sib effect is also observed with the b2 DNA of phage 434, which resembles that of lambda between -197 and att but diverges radically from it to the left of base -197.