Using the in vitro mixed transcription system (Kajitani and Ishihama (1983a, 1983b), we examined selective transcription of truncated DNA templates carrying lac(UV5), rrnE or rpsA promoters by RNA polymerase holoenzymes from pairs of wild-type parents and mutants with a mutation in one or more RNA polymerase subunit genes. The promoter selectivity of RNA polymerases from two sigma-subunit mutants carrying either rpoD2 or rpoD285 differed markedly from that of the respective wild-type enzymes. Both the parental RNA polymerases, however, exhibited abnormal promoter selectivity compared with holoenzymes from various wild-type E. coli strains. On the other hand, all the RNA polymerases from rpoB and/or rpoC mutants and the respective wild-type parents were similar, if not identical, in promoter selection at low temperature. At high temperature, however, RNA polymerases from mutants carrying rpoB2B7 and rpoC4, affecting the beta and beta' subunits, respectively, showed decreased transcription from the high-affinity slow-transcribable promoter rrnEp2 whereas the rpoC92 and rpoB906 X rpoC907 mutant enzymes both lost transcription activity from the strong promoter lacP(UV5). Taking all these observations together we conclude that not only the sigma subunit but also the beta and beta' subunits are involved in the recognition of promoters.