Functional specialization within the α-subunit of Escherichia coli RNA polymerase

The RNA polymerase core enzyme of Escherichia coli has the subunit composition α₂ββ′, and when combined with one of several alternative α-subunits (initiation-specificity) produces holoenzyme capable of all the steps of transcription. Dimerization of the α-subunit and association with the β-subunit trigger assembly of the core enzyme. Analyses of a set of deletion derivatives of rpoA (which encodes α) have indicated that as many as 94 carboxy-terminal amino acids (but not 153) can be removed without preventing assembly of core-like complexes in vitro. Detailed analyses of these deletion mutants have now been performed in vivo. α-Polypeptides truncated from the carboxy terminus to amino acid residues 235, 256 or 296 are assembled not merely into core, but also into holoenzyme-like complexes in vivo, and at least in the first two cases both of the two α-subunits can be replaced by the truncated versions. Nevertheless, none can complement rpoA^ts alleles for growth at 42°C. We conclude that the domain(s) of α essential for the assembly of RNA polymerase (at least the major holoenzyme species) are confined to the amino-terminal 235 amino acids, while some other essential function(s) require residues close to the carboxy terminus.