ROLE OF THE IMP OPERON OF THE STREPTOMYCES-COELICOLOR GENETIC ELEMENTSLP1 - 2 IMP-ENCODED PROTEINS INTERACT TO AUTOREGULATE IMP EXPRESSIONAND CONTROL PLASMID MAINTENANCE

The Streptomyces coelicolor genetic element SLP1 can exist either inte grated into the host chromosome or as an autonomously replicating plas mid. The integrated form of SLP1 includes a locus (imp, for inhibition of plasmid maintenance) that can act both in cis and in trans to prev ent propagation of SLP1 as an extrachromosomal replicon (S. R. Grant, S. C. Lee, K. Kendall, and S. N. Cohen, Mol. Gen. Genet. 217:324-331, 1989). We report here that a 1.8-kb Eco47III DNA fragment previously s hown to encode the Imp+ phenotype contains two genes (impA and impC) t hat must be expressed in cis to each other and whose products interact functionally and probably physically to interfere with SLP1 plasmid m aintenance and repress expression of the imp operon. Partial repressio n of the imp promoter (P(imp)), which is located immediately 5' of imp A, by the 29.7-kDa ImpA protein is enhanced by the impC gene product. Gel shift analysis indicates that ImpA binds to a 16-bp sequence locat ed within the DNA segment containing P(imp) and that ImpC interferes w ith this binding. Our data suggest that binding of ImpA to the P(imp) region mediates DNA looping in this region.