GROWTH RATE-RELATED REGULATION OF THE ILVGMEDA OPERON OF ESCHERICHIA-COLI K-12 IS A CONSEQUENCE OF THE POLAR FRAMESHIFT MUTATION IN THE ILVG GENE OF THIS STRAIN

In Escherichia coli K-12 the intracellular levels of threonine deamina se and transaminase B, products of ilvA and ilvE, respectively, in the ilvGMEDA operon, increase with increasing growth rates (S. Pedersen, P. L. Bloch, S. Reeh, and F. C. Neidhardt, Cell 14:179-190, 1978). How ever, the transcriptional activities of the upstream ilvp(G) and the i nternal ilvp(E) promoters do not increase. Therefore, the growth rate- related expression of this operon is not regulated at the level of tra nscription initiation, Unlike other wild-type E. coli strains, E. coli K-12 contains a polar frameshift mutation in the ilvG gene (R. P. Law ther, D. H. Calhoun, C. W. Adams, C. A. Hauser, J. Gray, and G. W. Hat field, Proc. Natl. Acad,.Sci, USA 78:922-925, 1981). In an E. coli K-1 2 (IVG(+)) derivative strain, where the reading frame of the ilvG gene is restored, no growth rate-related expression of the ilvGMEDA operon is observed, Thus, the growth rate-related expression of the ilvGMEDA operon in E, coli K-12 is the fortuitous consequence of the polar fra meshift mutation in the ilvG gene of this strain.