Control of gene expression in bacteriophage P22 by a small antisense RNA

The characterization in vitro of a newly discovered promoter {P,~} in the bacteriophage P22 i m m I region is described. P.,, is located within the ant gene and is directed toward the major i m m l promoter, P.~t. The entire intercistronic region between the P22 arc and ant genes (69 bp) is transcribed. The initiation and termination of sar (small antisense regulatory) RNA transcription are unusual. Frequent abortive initiation occurs in the presence of all four NTPs; RNA products 3-13 nucleotides in length are produced in about 15to 25-fold larger numbers than full-length transcripts. Termination of sar RNA synthesis occurs after transcription of the first and second Ts of a TTTA sequence following a region of hyphenated dyad symmetry. The effects of convergent transcription between P~t and P.~ were investigated on linear and supercoiled templates. Active transcription from P~.t interferes with full-length transcription from Psi,; several factors that interfere with P~t initiation (e.g., P~.t down-mutation, Mnt repressor protein, Arc repressor protein} result in indirect activation of sa t RNA synthesis. The sar R N A pairs rapidly with ant mRNA to form a stable stoichiometric complex. The location and properties of P.~, suggest an important regulatory function for sar RNA as a negative effector of ant expression. The results of Wu et al. (this issue} support this suggestion.