Synthesis of RepK of rolling circle plasmid pKYM is regulated by countertranscript and HU protein

Replication of rolling circle plasmid pKYM was regulated by RepK, a plasmid-encoded initiator protein, with HU protein and antisense RNA (copRNA) that block the expression of RepK. HU protein bound to the repK promoter in the presence of RepK protein and inhibited the transcription of repK mRNA. The copRNA would hybridize to repK mRNA and induce a stem-loop structure in which the repK Shine-Dalgarno sequence is sequestered by base pairing. Sequence substitution experiments demonstrated that this stem-loop not only inhibits translation but induces premature termination.