Products of nitrogen regulatory genes ntrA and ntrC of enteric bacteria activate glnA transcription in vitro: evidence that the ntrA product is a sigma factor.

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RESUMO

In enteric bacteria the products of two nitrogen regulatory genes, ntrA and ntrC, activate transcription of glnA, the structural gene encoding glutamine synthetase, both in vivo and in vitro. The ntrC product (gpntrC) is a DNA-binding protein, which binds to five sites in the glnA promoter-regulatory region and appears to activate transcription initiation. Using as an assay the stimulation of glnA transcription in a coupled in vitro transcription-translation system, we have partially purified the ntrA gene product (gpntrA). The following evidence is consistent with the view that gpntrA is a sigma subunit for RNA polymerase: (i) The gpntrA activity copurifies with the sigma 70 holoenzyme (E sigma 70) and core (E) forms of RNA polymerase through several steps but can be separated from them by chromatography on heparin agarose. (ii) After further purification by molecular sieve chromatography, the partially purified gpntrA fraction allows transcription of glnA from the same startpoint used in vivo; transcription is dependent on gpntrC and on added E. The gpntrA fraction does not allow transcription from promoters that we have used as controls, including lacUV5. E sigma 70 has the reverse specificity.

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