The bacteriophage P2 ogr gene encodes an 8.3-kDa protein that is a positive effector of P2 late gene transcription. The ogr gene is preceded by a promoter sequence (Pogr) resembling a normal Escherichia coli promoter and is located just downstream of a late transcription unit. We analyzed the kinetics and regulation of ogr gene transcription by using an ogr-specific antisense RNA probe in an Si mapping assay. During a normal P2 infection, ogr gene transcription starts from Pgr an intermediate time between the onset of early and late transcription. At late times after infection the ogr gene is cotranscribed with the late FETUD operon; the ogr gene product thus positively regulates its own synthesis from the P2 late promoter PF. Expression of the P2 late genes also requires P2 DNA replication. Complementation experiments and transcriptional analysis show that a nonreplicating P2 phage expresses the ogr gene from Pg, but is unable to transcribe the late genes.A P2 ogr-defective phage makes an increased level of ogr mRNA, consistent with autogenous control from Pogr. Transcription of the ogr gene in the prophage of a P2 heteroimmune lysogen is stimulated after infection with P2, suggesting that Pogr is under indirect immunity control and is activated by a yet-unidentified P2 early gene product during infection.Bacteriophage P2 belongs to a group of noninducible temperate phages of Escherichia coli that differ from the lambdoid phages in both gene organization and regulation (for reviews, see references 1 and 11). P2 early transcription is confined to the right half of the P2 chromosome (16) and initiates at two divergent promoters, P, and Pe, in the early control region (38). Transcription of the left half of the chromosome, where most of the phage morphogenesis genes are located, is induced later in infection (17). The P2 late genes are clustered into four transcription units based on the polar effects of amber mutations (29,46) (Fig. 1). Transcription of these operons depends on the host RNA polymerase (31), the P2 DNA replication genes A and B (17,28,29), and the P2 ogr gene (3,45). The four late operons are all expressed at the same time during a P2 infection, suggesting independent and coordinated transcription initiation from the late promoters (9,10,28). Late transcription initiates from sites that do not resemble promoters normally recognized by the host RNA polymerase (9, 10).The product of the P2 ogr gene is a 72-amino-acid basic protein (4,12,26,27)