DNA sequence of regulatory region for integration gene of bacteriophage λ

Abstract

The cII and cIII proteins specified by bacteriophage λ 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 λ 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.