Nucleotide sequence of the transcriptional repressor gene korB which plays a key role in regulation of the copy number of broad host range plasmid RK2.

The product of the korB gene of broad host range plasmid RK2 is one of at least two proteins which repress transcription of the essential replication gene trfA. We report here the nucleotide sequence of korB and the properties of its predicted polypeptide product KorB which has a molecular weight of 39,011 Da. KorB is likely to be a soluble protein with an overall net negative charge. However, consistent with a role in transcriptional regulation, there is a region with extensive homology to the alpha helix-turn-alpha helix motif of many DNA binding proteins. This region shows no significant homology to equivalent regions of the TrfB protein which is the primary transcriptional repressor of RK2 and which binds to an operator whose half sites show considerable homology to the half sites of the korB operator.