An operator associated with autoregulation of the repressor gene in actinophage phiC31 is found in highly conserved copies in intergenic regions in the phage genome.

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Previous reports have suggested that the repressor gene, c, of phiC31 is autoregulated and that likely operators are conserved inverted repeat sequences (CIRs1&2) located just upstream of the promoters, cp1 and cp2. Evidence is now presented that the CIRs 1&2 are indeed binding sites for one of the three inframe, N-terminally different protein isoforms of 42, 54 and 74 kDa produced by the c gene. A cp1-aphII fusion was repressed in a Streptomyces coelicolor A3(2) phiC31 lysogen and characterisation of an operator-constitutive (Oc) mutant showed a single mutation in CIR-1. CIR-1 containing fragments were retarded in electrophoresis gels by the 42 kDa repressor protein isoform and this retardation was inhibited by the addition of competing DNA fragments containing either CIR-1 or CIR-2. Using a combination of Southern blotting and analysis of available DNA sequence we also show that at least 18 copies of the CIRs are present throughout the phiC31 genome. Alignment of 9 CIR sequences showed that 8 contained a perfectly conserved 17 bp core whilst the exception had a single mismatch. The core includes a 16 bp inverted repeat (IR), and is usually part of a more extensive and less highly conserved palindrome. When superimposed on a previously derived transcription map of the early region, the CIRs lie in intergenic regions associated with transcription initiation and/or termination.

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