THE INT GENES OF BACTERIOPHAGES P22 AND LAMBDA ARE REGULATED BY DIFFERENT MECHANISMS

Bacteriophage P22 and lambda are related bacteriophages with similar gene organizations. In lambda the cII-dependent p(i) promoter is responsible for lambda int gene expression. The only apparent counterpart to p(i) in P22 is oriented in the opposite direction, and cannot transcribe the P22 int gene. We show that this promoter, called p(al), is active both in vivo and in vitro, and is dependent upon the P22 cII-like gene, called cl. We have also determined the DNA sequence of a 3.3 kb segment that closes the gap between previously reported sequences to give a continuous sequence between the P22 p(L) promoter and the int gene. The newly determined sequence is densely packed with genes from the p(L) direction, and the proteins predicted by the sequence show excellent correlation with the proteins mapped by Youderian and Susskind in 1980. However, the sequence contains no apparent genes in the opposite (p(al)) direction, and no additional binding motifs for the P22 c1 protein. We conclude that int gene expression in P22 is regulated by a different mechanism than in lambda.