Transcription in the trfA region of broad host range plasmid RK2 is regulated by trfB and korB.

• Transcription at various points in the trf A region of broad host range plasmid RK2 has been analysed by measuring expression of the galK gene inserted at EcoRI sites introduced previously by TB1723 transposition mutagenesis. Rightward transcription (anti-clockwise on RK2) probably from a single promoter, proceeds across two open reading frames coding for a 13 kD polypeptide of unknown function, and the trf A gene, which provides a protein(s) essential for plasmid replication. This transcription is not auto-regulated by the products of either open reading frame and is also not subject to significant attenuation prior to the end of the trfA open reading frame. Leftward transcription appears to be directed by at least two well separated promoters, the more leftward being three to four times stronger than the more rightward. Rightward, but not leftward, transcription is repressed about 9-fold by the trfB locus of RK2 alone (so far not separable from the loci korA and korD) in trans while the combination of the korB and trfB loci in trans represses both rightward transcription (about 100-fold) and leftward transcription (the stronger activity by 10 to 15-fold). Regulation of these operons is therefore qualitatively different. The kilD locus in the trfA region, which is suppressed by korD (trfB) is thus probably part of the rightward (trfA) operon, while leftward transcription may represent the start of an operon containing kilB. The results suggest that RK2kor loci act by repressing transcription of kil loci and that the kil and kor control circuits may be part of an interlocking system of RK2 genes involved in replication and stable maintenance.

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