Transcriptional Organization of the Typhimurium Phage P22 ORFan Locus.

Many phage genes lack sequence similarity to any other open reading frame (ORF) in current databases. These enigmatic ORFan genes can have a tremendous impact on phage propagation and host interactions but often remain experimentally unexplored. We previously revealed a novel interaction between phage P22 and its Typhimurium host, instigated by the ORFan gene (for phage P22 encoded instigator of expression) and resulting in derepression of the host operon. The gene is highly expressed in phage carrier cells that harbor a polarly located P22 episome that segregates asymmetrically among daughter cells. Here, we discovered that the locus is fitted with a weak promoter, has an exceptionally long 5' untranslated region that is instructive for a secondary mRNA species, and has a 3' Rho-independent termination loop that is responsible for stability of the transcript.