Evidence for Two Classes of Prereplicative Cistrons*

The antibiotic rifampicin has been used in an investigation of the temporal regulation of gene expression during bacteriophage T4 infection of Escherichia coli. We have asked at what times after infection the transcription of specific cistrons becomes rifampicin insensitive. Gene expression has been studied using analyses of proteins on sodium dodecyl sulfate-acrylamide gels; thus translation has been used to assay for the appearance of specific transcripts. Two types of promoters account for most prereplicative transcription; early promoters are recognized immediately after infection, whereas quasi-late promoters are recognized only after a delay of 14 min. Those transcripts previously defined as immediate early and delayed early RNAs initially appear to be under the control of early promoters; however, the quasi-late promoters may provide a second mode of regulation for several of those transcripts.