DNA sequence of regulatory region for integration gene of bacteriophage lambda

The cII and cIII proteins specified by bacteriophage lambda direct the lysogenic response to infection through the coordinate establishment of repression and integration of the viral DNA. The regulatory activity of cII/cIII involves positive regulation of two promoter sites: the p(E) promoter, turning on expression of the cI protein that maintains lysogeny, and the p(I) promoter, activating synthesis of the Int protein for integrative recombination. Regulation of the p(I) promoter provides for differential expression of the Int protein with respect to the excision-specific Xis protein from the closely linked int and xis genes. We have determined the DNA sequence of the p(I) promoter region for wild-type lambda DNA and for two classes of mutations: intc mutations, which result in a high rate of Int synthesis in the absence of cII, and deletion mutations, some of which eliminate cII-activated expression of the int gene. We find a sequence with considerable homology (11 of 15 bases) to a "typical" (computer-generated) promoter sequence, adjacent to a region with striking homology (11 of 14 bases) to part of the p(E) promoter region. This presumed p(I) sequence overlaps the start of the xis gene and includes the site of two intc point mutations. A cII-insensitive xis(+) deletion partially removes the proposed p(I) sequence; a deletion that leaves the p(I) sequence intact but terminates 21 bases upstream does not interfere with cII activation of the int gene. From our results and the analysis of the p(E) region, we suggest that cII acts in the promoter -35 recognition region to facilitate binding by RNA polymerase at the -10 interaction region. Differential expression of the int and xis genes results because the p(I) transcript lacks the initiation codon for Xis protein synthesis.