Control of phage P22 tail protein expression by transcription termination

The structural gene for Salmonella bacteriophage P22 tail protein, gene 9, is separated from the remainder of the P22 late operon genes by the immI region. Early transcription of immI gene ant, which is immediately promoter proximal to gene 9, occurs in the same direction as late gene transcription but does not enter gene 9 coding sequences (Susskind & Youderian, 1982). We have cloned gene 9 and surrounding sequences into pBR322 and subsequently positioned lac UV5 promoters at varying distances before the start of gene 9 by DNA manipulations in vitro. Using an in vitro phage assembly assay to measure in vivo expression of tail protein from these plasmids and in vitro transcription reactions to measure transcriptional template activity of DNA fragments isolated from these plasmids, we have identified a region of DNA between gene ant and gene 9 that behaves as a transcription termination signal. The DNA sequence of this region shows hyphenated dyad symmetry followed by a run of seven thymine residues on the coding strand. This sequence can be drawn in a potential stem-and-loop secondary structure similar to known rho-independent transcription termination signal sequences. We discuss the role of this transcriptional terminator sequence in gene 9 expression and the early to late transcriptional switch in the P22 infection cycle.