Influence of the expression of a short RNA fragment, complementary to the E. coli 23S rRNA sequence from nucleotides 735 to 766, on a translational read-through at UGA, UAA, and UAG nonsense codons in the cat gene. Cells carrying the cat gene with one of the three nonsense mutations were transformed with either pPOT1 vector or pA750. Cultures were grown with (+) or without (−) IPTG, total cellular proteins were separated in SDS gels and blotted, and CAT protein was detected with CAT-specific antibodies (see text for details). The pACYC184 lane shows CAT protein expressed in cells transformed with pACYC184 plasmid (wild-type cat); the CAT lane shows purified CAT protein.