MUTATIONS AFFECTING 2 ADJACENT AMINO-ACID-RESIDUES IN THE ALPHA-SUBUNIT OF RNA-POLYMERASE BLOCK TRANSCRIPTIONAL ACTIVATION BY THE BACTERIOPHAGE-P2-OGR PROTEIN

The bacteriophage P2 ogr gene product is a positive regulator of trans cription from P2 late promoters. The ogr gene was originally defined b y compensatory mutations that overcame the block to P2 growth imposed by a host mutation, rpoA109, in the gene encoding the alpha subunit of RNA polymerase. DNA sequence analysis has confirmed that this mutatio n affects the C-terminal region of the alpha subunit, changing a leuci ne residue at position 290 to a histidine (rpoAL290H). We have employe d a reporter plasmid system to screen other, previously described, rpo A mutants for effects on activation of a P2 late promoter and have ide ntified a second allele, rpoA155, that blocks P2 late transcription. T his mutation lies just upstream of rpoAL290H, changing the leucine res idue at position 289 to a phenylalanine (rpoAL289F). The effect of the rpoAL289F mutation is not suppressed by the rpoAL290H-compensatory P2 ogr mutation. P2 ogr mutants that overcome the block imposed by rpoAL 289F were isolated and characterized. Our results are consistent with a direct interaction between Ogr and the alpha subunit of RNA polymera se and support a model in which transcription factor contact sites wit hin the C terminus of alpha are discrete and tightly clustered.