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.