Substitution of a single bacteriophage T3 residue in bacteriophage T7 RNA polymerase at position 748 results in a switch in promoter specificity.

The bacteriophage T3 and T7 RNA polymerases (RNAP) are closely related, yet exhibit high specificity for their own promoter sequences. In this work the primary determinant of T7 versus T3 promoter specificity has been localized to a single amino acid residue at position 748 in the T7 RNAP. Substitution of this residue (Asn) with the corresponding residue found in T3 RNAP (Asp) results in a switch in promoter specificity, and specifically alters recognition of the base pairs (bp) at positions -11 and, possibly, -10 in the promoter. A complementary mutation in T3 RNAP (T3-D749N) results in a similar switch in promoter preference for that enzyme. The hierarchy of bp preference by the mutant and wild-type enzymes for bp at -10 and -11, and the results of previous experiments, lead to a model for specificity in which it is proposed that N748 in T7 RNAP (and D749 in T3 RNAP) make specific hydrogen bonds with bases at -11 and -10 on the non-template strand in the major groove. The specificity determining region of T7 RNAP does not appear to exhibit homology to any known sequence-dependent DNA binding motif.