Low activity of -galactosidase in frameshift mutants of Escherichia coli

16 lac frameshift mutants induced by an acridine derivative, ICR-191D, in E. coli are leaky for beta-galactosidase activity. Activities of all mutants differ from each other and from the wild type in their stability to thermal denaturation. The leakiness is under ribosomal control, since it is strongly reduced by strA restrictive mutations and is restored by ram mutations that reverse restriction. Addition of streptomycin during growth has an effect similar to the presence of the ram mutation. These ribosomal alterations do not modify the thermal stability of the enzyme.It is suggested that the leakiness is due to an infrequent 2- or 4-base reading close to the frameshift mutation site. The possibility that not only the ribosome, but also the reading context in the messenger, plays a role in securing code fidelity is discussed.