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A second RNA-polymerase can bind specifically to the bla promoter of Tn3, repressing transcription initiation.

We showed earlier that the region of the bla promoter of Tn3 protected by the RNA-polymerase (RNAP), has the normal size (about 60bp) at RNAP/promoter molar ratio r less than or equal to 2, but rises to about twice this extent as r increases. We confirm here that the species corresponding to normal and extended footprint distinguish by their electrophoretic mobilities. Furthermore, inspection of the complexes by electron microscopy confirms that at r greater than 2, the bla promoter can bind specifically a second RNAP particle, as compared to the 1:1 complex observed at r less than or equal to 2. At r greater than 2, the ability of the bla promoter to initiate transcription in vitro is repressed when compared to the complex 1:1 obtained at r less than or equal to 2. The unexpected decrease in initiation efficiency as the concentration of RNAP particles is increased, together with the striking sequence homology of the bla promoter with promoters of stable RNA, suggest that in vivo, this promoter could be regulated by growth rate.