Transcription in vitro of stringently controlled Escherichia coli genes by purified RNA polymerase holoenzyne is inhibited by guanosine tetraphosphate (ppGpp). In order to examine possible role of ω factor in this ppGpp sensitivity, RNA polymerases with or without the ω factor were reconstituted and tested for their ppGpp sensitivity using an in vitro mixed transcription system. RNA polymerase lacking the ω factor was found virtually insensitive to ppGpp but the addition of a purified ω factor restored the ppGpp sensitivity of this ω -free RNA polymerase. These results raise a possibility that the ω factor is a regulatory protein of RNA polymerase and is involved in the ppGpp-mediated alteration of the promoter selectivity.
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