Regulation of protein synthesis in bacteriophage λ restoration of gene expression in λN− strains by mutations in the cro gene

Abstract

The N protein of bacteriophage λ is required to extend mRNA transcription beyond rho termination sites [ Roberts, J. W. (1969). Nature (London) 223, 480–4821. In the absence of a functional N protein very little transcription takes place. The present study describes a unique pattern of restoration of gene expression in λN − strains by mutations in gene cro. While a λN − cro − phage does not grow on a nonpermissive Escherichia coli host, it follows the lysogenic pathway efficiently. This phage strain was found to synthesize an appreciable level of gene products of the two early λ operons as well as the cI repressor and Int protein (integrase). The experiments were conducted by infecting ultraviolet-irradiated cells with a variety of phage mutants and analyzing the synthesis of specific proteins by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. Previous identifications of various early and late gene products were confirmed. Gene expression in the λN − cro − strains suggests a direct intimate interaction between these regulators. The results also indicate that, in the absence of N protein, transcription terminates prematurely following an exponential mode of decay. In addition, the presence of a previously hidden termination site (t L2) located within the early leftward (P L) operon is suggested.