The expression of a region of the bacteriophage T4 genome is being studied as a model for examining developmental gene regulation. Throughout infection, T4 uses the host transcriptional apparatus to transcribe its DNA, but with time different regions of the T4 genome are transcribed. To determine signals and factors that regulate this transcription, the expression of the T4 genes uvsX (recombination protein), 40 (stimulates head formation), and 41 (primase-helicase component) have been examined by nuclease Sl protection experiments and Northern analyses. Changes in the transcription pattern of the uvsX-40-41 region are programmed with phage development. Early in infection a heterogeneous population of transcripts is observed, having major 5' starts about 900 and 200 bases upstream of uvsX. The bulk of these RNAs continue through 40 and into 41, but a portion (about 1/4) stop just downstream of uvsX (within gene 40). Later in infection a 5' start closer to uvsX (about 50 bases upstream) is observed, in addition to the ends seen earlier, and a significant fraction of the uvsX RNA ends at the stop. Thus, as infection proceeds, the level of 40, 41 mRNA decreases relative to that of uvsX. Analysis of uvsX-40-41 transcripts expressed by a plasmid with this region reveals different 5' ends from those after T4 infection, but significant utilization of the RNA stop downstream of uvsX. Thus, T4 infection is needed for the 5' starts, while the host can produce the RNA stop. To help determine what host factor(s) are needed for this stop, transcripts were examined after infection of the E. coli transcription termination (rho) mutant, rho026. Previous work has suggested that in rho026, transcription termination may be enhanced, resulting in less expression of regions downstream of a rho-dependent termination site (Stitt et al. (1980) J. Virol. 35, 775). My transcription mapping indicates that infection of rho026 results in several-fold less 40,41 mRNA than infection of the rho+ parent, because of a 2.5-fold greater use of the RNA stop and less transcription overall. Thus, rho factor either acts to terminate transcription at the stop or influences other factors needed for the regulation of this site.
