Genetic studies on the beta subunit of Escherichia coli RNA polymerase. I. The effect of known, single amino acid substitutions in an essential protein.

The use of five different nonsense suppressors, in conjunction with a collection of 95 independent, spontaneously-occurring amber mutants affecting expression of rpoB, allows the generation of a maximum of 475 potential variants of the beta subunit of E. coli RNA polymerase, each carrying a known amino acid substitution at a particular site. The effect of these amino acid exchanges has been investigated in vivo. A significant majority (363/475) of substitutions lead to cellular death and altered properties--temperature sensitivity, apparently altered transcription termination and a changed stringent response--indicating that RNA polymerase function (unlike that of dispensable proteins) is extremely sensitive to such single site changes.