Transcription Termination Regulates Gene Expression

Abstract

INTRODUCTION
The expression of many microbial genes is controlled according to the “operon” model proposed by Jacob and Monod (1961 Jacob and Monod (1962); regulation occurs at the level of initiation of transcription. The control is achieved by stimulating and/or inhibiting the initiation of transcription (Beckwith and Rossow 1974). The operons behave as transcriptional units in the chromosome, implying that transcription, once initiated, reads through the entire operon and finally terminates at the promoter-distal end. Recently, however, transcription termination signals have been shown to be present also within operons (de Crombrugghe et al. 1973; Shimizu and Hayashi 1974). It is now clear that transcription termination can be regulated so as to control the expression of genes or even operons located distal to a termination signal. This type of control was first shown in bacteriophage λ. In what follows we shall present evidence for such a control system and propose molecular mechanisms by which this control is accomplished.

RESULTS AND DISCUSSION
Control of Gene Expression in Phage λ
When phage λ infects E. coli or when cells lysogenic for a λ prophage are induced, a regulated sequence of phage gene expression takes place. First, the “immediate-early” genes N and tof are transcribed from the two λ promoters p_L and p_R, respectively (for review, see Echols 1971; see also the genetic map of prophage λ in Figure 1). Gene N is transcribed from the l strand of λ DNA and tof is transcribed from the r strand (Skalka, Butler and Echols 1967; Kourilsky et al....