The immI operon of P1 contains the genes c4, icd (formerly called orfx), and ant which are constitutively transcribed in that order from a single promoter, P51b. C4 is an antisense RNA which is processed from the precursor transcript. C4 RNA acts as a translational repressor of icd, thereby also inhibiting antirepressor (ant) synthesis. We have cloned the icd and the overlapping icd and ant genes. We show, by means of plasmid deletion analysis, that icd is translationally coupled to ant. An internal in-frame deletion of icd making up 65% of the codons still allows antirepressor synthesis at a reduced rate, indicating that a functionally active icd gene product is dispensable for ant expression. We identify the product of the icd gene as a 7.3-kDa protein which interferes with cell division. The results suggest that constitutive expression of icd, in the absence of a functionally active antirepressor, prevents P1 lysogen formation because of its detrimental effect on the host cell.