Gene expression in the cos region of the Streptomyces temperate actinophage phi C31

A transcription map of a 5.12 kb region containing the cos ends of actinophage phi C31 was determined using RNA prepared from induced and uninduced cultures of the temperature-sensitive lysogen, Streptomyces coelicolor A3(2) (phi C31cts1). In induced cultures, RNA synthesis was detected only late in the lytic cycle. A late operon initiated downstream of the int gene on the right-hand end of the genome traversed the cos ends and extended at least 3.6 kb along the left-hand end. Shorter, possibly processed, mRNAs were also present. The map was superimposed on the DNA sequence of 2.8 kb of the region, part of which had been determined previously and part of which is presented here. The late-expressed transcripts contained a tRNA(Thr)-like gene and four ORFs (1, 2, 3 and 5) detected on the basis of codon bias. Analysis of the putative protein products showed that one of the ORFs could encode a lysis protein and at least one may be involved in DNA maturation. Transcription mapping of RNA from uninduced cultures demonstrated a 620 nt transcript, xRNA1, of ORF6. So far this is the only gene in phi C31 found to be expressed right to left with respect to the standard map of phi C31; its function during lysogenic growth could not be deduced from database searches.