STUDIES OF BACTERIOPHAGE-T7 RNA-POLYMERASE FORMS WHICH CONTAIN POINT SUBSTITUTIONS IN A REGION OF FUNCTIONALLY ESSENTIAL AMINO-ACID-RESIDUES

A study was made of the effect of point substitutions for functionally essential amino acid residues of bacteriophage T7 RNA polymerase (T7R P) on its secondary and tertiary structures, as well as on its functio nal activity. The substitution for the residue at position 631, advers ely affecting the enzyme activity, was shown to be attended by signifi cant variations in the secondary structure (a --> beta transition). Su bstitution of Leu for the Lys-172 residue is attended by variations in both secondary and tertiary structures, whereas deletion of this resi due along with the neighboring Arg-173 does not cause similar variatio ns. An examination of the effect of mutations in the 631-632 region sh ows that substitutions for these residues result in neither disturbanc e of the promoter binding by the enzyme nor premature termination of R NA chains, but affect mainly the capacity of T7RP for phosphodiester b ond formation at the elongation and, probably, termination steps.