The effect of low substrate concentrations on the extent of productive RNA chain initiation from T7 promoters A1 and A2 by Escherichia coli RNA polymerase

The extent of productive RNA chain initiation in vitro by Escherichia coli RNA polymerase holoenzyme from the bacteriophage T7 A1 and A2 promoters on purified T7 DNA templates has been investigated at very low concentrations of the ribonucleoside triphosphate substrates. As the concentration of ribonucleoside triphosphates in the reaction is decreased from 10 to 1 micro M, the extent of productive RNA chain initiation at these promoter sites drops precipitously at about 3 micro M. At 1 micro M substrate concentration, productive chain initiation from the A1 promoter does not occur even after extended incubation although the dinucleoside tetraphosphate pppApU is produced at a significant rate under these conditions. The reason for the inability of RNA polymerase to carry out productive RNA chain initiation at 1 micro M substrate concentration is not yet understood. The phenomenon is not due to substrate consumption, enzyme inactivation, or a requirement for a nucleoside triphosphatase activity in the reaction. The possibility is raised that there are additional nucleoside triphosphate binding sites on E. coli RNA polymerase which play some role in the process of productive RNA chain initiation.