Transcriptional control of two gene subclusters in the tRNA operon of bacteriophage T4.

In bacteriophage T4 DNA, transcription units recognized in vitro by host RNA polymerase consist of promotor-proximal 'immediate early' (IE) genes and promotor-distal 'delayed early' (DE) genes separated from each other by rho-dependent transcription terminators. In vivo, the transition from IE to DE transcription requires phage-specific protein synthesis and can be prevented by chloramphenicol (CAM). Most of the information about IE/DE transition has been obtained by hybridizaton analyses of mixtures of RNA species synthesized simultaneously on several T4 transcription units (for review see ref. 3). A useful model for the study of T4 gene expression at the level of primary transcripts and individual gene products is provided by the T4 tRNA operon, a cluster of genes coding for eight T4-specific transfer RNAs and two stable RNAs (species 1 and 2) of unknown function (Fig. 1). The 10 genes of the tRNA operon are arranged in two subclusters (I and II) with a promotor located about 1 kilobase pair upstream. The primary transcripts and the final gene products of this region have been identified and isolated. Moreover, this genetic region was recently cloned and a part of it sequenced. We describe here the expression of T4 tRNA genes in vivo and in vitro in terms of the IE/DE concept and demonstrate that the two subclusters of the tRNA operon are subject to different modes of control.