In broad host-range plasmid RK2, korA function prevents the lethal effect of kilA on Escherichia coli host cells and inhibits expression of trfA, the essential replication gene. From gene fusion and promoter replacement studies, we determined that control of kilA is also mediated at the level of gene expression and that the target resides in the kilA promoter region. The nucleotide sequence of this region shows the same two operator-like palindromes present in the previously sequenced promoters of trfA and korA. One of the palindromes (5'-GTTTAGCTAAAC-3') at the -10 position is sufficient to confer sensitivity to korA function. The presence of the same sequences in the korA promoter region suggested that korA might also regulate its own expression. Using the structural gene for chloramphenicol acetyltransferase (cat) fused to the korA promoter, we found that korA gene expression is indeed autoregulated. The results show that korA gene product is very likely a repressor that negatively regulates expression of at least three different genes by interacting with an operator-like sequence in their promoter regions. Coordinate regulation of host-lethal gene kilA and essential replication gene trfA by a common mechanism also supports our hypothesis that these genes are functionally related.