Establishment of Repressor Synthesis

INTRODUCTION mRNA encoding the c I repressor of bacteriophage λ can be initiated at either of two promoters, which function at different stages in the λ life cycle. In the lysogenic state, repressor transcription initiates from the p RM promoter, which lies immediately adjacent to the c I gene (Fig. 1). This promoter is positively regulated by the c I-gene product and both structurally and functionally overlaps with the promoter-operator signals ( p R o R ) controlling the major rightward operon of λ . In contrast, when λ infects a sensitive cell, repressor transcription originates from the p RE promoter, which lies several hundred base pairs upstream of the c I gene in the λ y region (Fig. 1). This promoter structurally overlaps the region encoding the aminoterminal amino acid sequence of the c II gene. Transcription from p RE is also positively regulated and, in this case, the activator is the c II protein. The c II protein also activates transcription from p I , the promoter for the λ integrase gene. Thus, the c II protein coordinately regulates the two transcription units required for the lysogenic response and thereby acts as a major determinant in controlling the balance between lytic and lysogenic development. The c II activity is itself regulated at multiple levels (transcription, translation, processing, stability, and so on), and a variety of phage and host-encoded functions take part in this regulation. In this paper we examine the p RE transcription unit, with particular emphasis on the p RE promoter signal and its mode of activation by c II protein. We examine the c II protein both genetically and biochemically and...