Physical mapping of the early region of bacteriophage T7 DNA

Abstract

Heteroduplex mapping of bacteriophage T7 deletions, together with genetic and electrophoretie analysis of T7 RNAs and proteins, has produced a detailed physical map of a substantial segment of T7 DNA, including most of the early region. Position in the T7 molecule is given as percentage of the length of wild type T7 DNA, measured from the left end. The signal that stops transcription by Escherichia coli RNA polymerase at the end of the early region is located near position 20.2. The five early messenger RNAs seem to occupy fully the region between 1.8 and 20.2, with little if any gap between molecules, and the ends of these RNAs have been mapped at positions 1.8, 3.4, 8.0, 15.5, 17.1 and 20.2. When the stop signal is deleted, transcription by host RNA polymerase continues on and the next RNA ends at position 30.1. The positions of some early T7 proteins, as well as one protein to the right of the stop signal, have also been determined. The mapping of the 0.3 and 0.7 proteins is not straightforward, and may indicate that these proteins are somehow processed during or after synthesis. The DNA between positions 2.8 and 8.1, and between positions 15.2 and 23.5, is not essential for growth of the phage, since it can be eliminated by deletions. Genes 0.7, 1.1 and 1.3 can be deleted entirely, as can the usual origin of replication of T7 DNA, determined by Dressier et al. (1972) to be near position 17. The molecular weights of the early T7 RNAs range between 208,000 and 975,000 and give a convenient spread to use as standards for electrophoresis of RNAs in polyacrylamide gels.