Site-specific recombination functions of bacteriophage λ: DNA sequence of regulatory regions and overlapping structural genes for Int and Xis

Abstract

Site-specific recombination in bacteriophage .lambda. is mediated by 2 phage-encoded proteins, Int and Xis. The structural genes encoding these proteins are located immediately to the right of their site of action, the phage att site. The DNA sequence for both the structural and regulatory regions of these genes was determined. The location and reading frame of the xis gene were ascertained by sequence comparisons with the b538 deletion (that ends within xis) and with the xis6 amber mutation. From the DNA sequence Xis has a MW of 8630; it is rich in basic amino acids with lysine and arginine comprising 25% of the 72 amino acids. Identification of the int reading frame was also unambiguous. From the DNA sequence, Int has a MW of 40,330; of the 356 amino acids, 69 are basic and 46 are acidic. In the NH2-terminal portion of Int, 35% of the first 20 amino acids are basic. The site-specific recombination functions form a very tight cluster (att-int-xis) on the .lambda. chromosome. The combined protein-encoding sequences of xis and int start 1347 base pairs, and terminate 84 base pairs, from the center of the phage att site. The 2 genes overlap one another by 20 base pairs (xis is upstream of int) and a possible means of controlling the relative synthesis rates of Int and Xis at the level of translation is proposed. Control at the level of transcription is also considered. The mutation intc226 leads to constitutive production of Int, independent of cII/cIII activator proteins normally required for transcription from the pI promoter. This mutation is the result of a single base change (in the fMet codon of the xis gene) that generates an improved promoter heptamer sequence. This result, in conjunction with comparisons with other promoter sequences and other sequences responding to cII/cIII action, leads to a tentative identification of the pI promoter and site of cII/cIII action.