Bacteriophage P22 gene 23 product acts preferentially in cis.

The expression of the P22 late operon was measured while the activator of the late operon, the product of gene 23, was provided in cis or in trans. It was found that expression of the late operon, assayed from a late-gene-lacZ gene fusion, was reduced by more than twofold when the only functional copy of gene 23 was present in trans.