A transcriptional map of the bacteriophage SP01 genome: II. The major early transcription units

Abstract

The genome of the Bacillus subtilis phage SP01 contains a 12.6-kilobase-pair terminal redundacy. The transcription units in this region, which are utilized in vitro by B. subtilis RNA polymerase holoenzme, have been mapped. In vitro transcripts were separated and characterized by gel electrophoresis. Since there is such a large number of promoters in this region, it was necessary to employ conditions of transcription under which only subsets of all the transcripts would be made. Selective synthesis was achieved by transcription of restriction fragments containing a small number of promoters and by dinucleotide priming. Transcription units were mapped by observing the shortening of transcripts that resulted from restriction enzyme cleavage of the template. The mapping indicates the presence of two blocks of overlapping transcription units, oriented so as to yield convergent transcription toward a bidirectional termination region. The frequency of readthrough at the bidirectional terminator is low. Several partial termination sites of differing efficiences have also been identified, including two that are active only at low ribonucleoside triphosphate concentrations. We also describe an effect of NaCl concentration on the migration of RNA in gels, which is specific for only two of the SP01 early transcripts. This effect may indicate unusual secondary structures for these two RNA molecules.