THE ALPHA-SUBUNIT OF RNA-POLYMERASE AND TRANSCRIPTION ANTITERMINATION

The N gene product of coliphage lambda, with a number of host proteins (Nus factors), regulates phage gene expression by modifying RNA polym erase to a form that overrides transcription-termination signals. Muta tions in host nus genes diminish this N-mediated antitermination. Here , we report the isolation and characterization of the rpoAD305E mutati on, a single amino acid change in the carboxy terminal domain (CTD) of the alpha subunit of RNA polymerase, that enhances N-mediated antiter mination, A deletion of the 3' terminus of rpoA, resulting in the expr ession of an alpha subunit missing the CTD, also enhances N-mediated a ntitermination and, similar to rpoAD305E, suppresses the effect of nus mutations, Thus, the N-Nus complex may be affected through contacts w ith the CTD of the alpha subunit of RNA polymerase, as is a group of r egulatory proteins that influences initiation of transcription. What d istinguishes our findings on the N-Nus complex from those of previous studies with transcription proteins is that all of the regulators char acterized in those studies bind DNA and influence transcription initia tion; whereas the N-Nus complex binds RNA and affects transcription el ongation, A screen of some previously identified rpoA mutations that i nfluence transcription activators revealed only one other amino acid c hange, L290H, in the CTD of the a subunit, that influences antitermina tion. Although our results provide evidence that interactions of the a subunit of RNA polymerase must be considered in forming models of tra nscription antitermination, they do not provide information as to whet her the interactions of a that ultimately influence antitermination oc cur during initiation or during elongation of transcription.