T3 × T7 phage crosses leading to recombinant RNA polymerases

Abstract

THE related virulent bacteriophages T3 and T7 code for RNA polymerases which are specific for promoters located on the DNA of these phages^1,2. These single peptide chain polymerases—the products of gene 1 (refs 1 and 3)—are genetically homologous but can be distinguished by their template specificity and their molecular weight. The T3 RNA polymerase transcribes T7 DNA at only about 10% of the rate at which T3 DNA is transcribed, and the T7 RNA polymerase transcribes T3 DNA at about 60% the homologous rate²; and, by direct comparison with the T7 polymerase, the molecular weight of the T3 polymerase was found to be about 5% lower than that of the corresponding T7 protein^4,5 (whose molecular weight has been estimated at 107,000 (ref. 1), 110,000 (ref. 2), 100,000 (ref. 6), and 108,000 (ref. 4). Since phages T3 and T7 can be crossed⁷, the differences between the T3 and T7 RNA polymerases provide a model system for studies on the correlation between genetic constitution and functional specificity. Here we report some initial experiments in this direction.