Defective assembly of ribonucleic acid polymerase subunits in a temperature-sensitive alpha-subunit mutant of Escherichia coli

Abstract

The subunit assembly of RNA polymerase was investigated for the temperature-sensitive Escherichia coli strains carrying the mutation rpoA101 or rpoA112 in the gene encoding its alpha subunit, In cells carrying rpoA112, the sequential assembly of enzyme subunits is blocked at an early step, i.e., either the dimerization of altered alpha subunit or the subsequent association of altered alpha dimer with beta subunit. As a result, the unassembled free alpha subunit accumulates and the unassembled beta and beta' subunits are degraded rapidly, in particular at a nonpermissive temperature. The assembly defect is accompanied by an overproduction of enzyme subunits apparently due to the decrease in the concentration of repressor holoenzyme involved in the autogenous regulation. These results together with the previous observations [Ishihama, A., Shimamoto, N., Aiba, H., Kawakami, K., Nashimoto, H., Tsugawa, A., & Uchida, H. (1980) J. Mol. Biol. 137, 137-150] indicate that the temperature-sensitive growth of rpoA112 mutants is attributed to the assembly defect of RNA polymerase as well as to the thermolability of assembled polymerase. In contrast, the altered alpha subunit in a mutant carrying rpoA101 is assembled into the polymerase structure as efficiently as in wild-type cells; nevertheless, both beta and beta' subunits are rapidly degraded in this mutant. This indicates that the mutant polymerase is structurally different from the metabolically stable wild-type enzyme. Thus, the ts character of rpoA101 mutant is explained by the alteration in the structure and function of assembled RNA polymerase.