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Activation of P2 late transcription by P2 Ogr protein requires a discrete contact site on the C terminus of the alpha subunit of Escherichia coli RNA polymerase.

Bacteriophage P2 late transcription requires the product of the P2 ogr gene. Ogr-dependent transcription from P2 late promoters is blocked by certain point mutations affecting the alpha subunits of the host RNA polymerase. An alanine scan spanning the putative activation target in the alpha C-terminal domain (alphaCTD) was carried out to identify individual residues essential for Ogr-dependent transcription from P2 late promoters. In addition, the effects of alanine substitutions in the regions of the alphaCTD previously reported to affect CAP-dependent activation of the lac promoter and UP-element DNA binding were examined. Residues E286, V287, L289 and L290 in helix 3, and residue L300 at the beginning of helix 4, define a surface-exposed patch on the alphaCTD important for Ogr-dependent activation. These residues, adjacent to the recently identified DNA-binding determinants, constitute a newly identified activation surface for protein:protein contact. Alanine substitutions at some of the residues that affect UP-element DNA binding also impaired activation. This suggests that upstream DNA-alpha contacts, in addition to alpha-Ogr contacts, may be important in P2 late transcription. Other residues implicated in the interaction of alpha with CAP are not required for activation by Ogr, consistent with previous genetic evidence suggesting that these activators contact different sites on the alphaCTD.