Regulation of the bacteriophage Mu gem operon.

Thegemoperon of bacteriophage Mu, responsible for the complex phenomenon of phage conversion, is included in the so called "semiessential early" region of phage DNA. Unlike the other early genes of the phage which are transcribed from the pepromoter, expression of thegemoperon is driven by its own promoter, which escapes the control of the repressor. In fact, the transcript corresponding togemwas detected in immune lysogens by using a combined reverse transcription and a subsequent amplification of the resulting cDNA. The transcription initiation site from pgemwas determined by primer extension mapping experiments and localized at 8217 bp from the left end of phage DNA. Two elements which could perform the negative control ofgemwere also identified. The first is a phage product, GemB, which presumably interferes withgemexpression at a posttranscriptional level, whereas the second is a structural element, an inverted repeat immediately downstream of pgem, which acts as a terminator for the transcripts starting from pe. These transcripts could regulategemexpression by interfering with the initiation of transcription from pgem.