The bacteriophage T4 genes uvsX (recombination protein), 40 (stimulates head formation), and 41 (DNA replication protein, part of the primase-helicase) are located together on the T4 genome (5'----3' uvsX-40-41). Previous analyses have indicated that all three proteins are expressed within 5 min after infection and that the level of 41 protein is less than that of uvsX. The mapping of transcripts from this region (reported here) shows that this expression arises from polycistronic messages detected between 2-4 min after infection, a time when phage-encoded factors are beginning to alter the host transcriptional apparatus. Major RNA 5' ends, 900 and 200 bases upstream of uvsX, show homology with previously deduced T4 transcription sites dependent on the T4 transcription factor motA (Guild, N., Gayle, M., Sweeney, R., Hollingsworth, T., Modeer, T., and Gold, L. (1988) J. Mol. Biol. 199, 241-258). Analysis of the 3' end of uvsX RNAs shows that initially most transcripts extend through gene 40 and 41, although approximately equal to one-fourth end just past uvsX (within gene 40). Later, more of the uvsX messages are monocistronic, having 5' ends close to the gene (200 and 55 bases upstream) and having the 3' end within gene 40. Thus, during infection the level of 41 RNA is lowered relative to uvsX message. Mapping of RNA expressed from an uvsX-40-41 plasmid in an uninfected cell gives 5' ends 700, 450, and 55 bases upstream of uvsX, i.e. positions different from those during T4 infection. This indicates that infection significantly changes the 5' ends for uvsX RNA, either by altering transcription initiation or RNA processing sites. In contrast, the majority of the uvsX RNAs expressed by plasmid in the uninfected cell do end at the stop mapped during infection. Thus, the host alone can produce this 3' end.