Abstract
In Escherichia coli K-12 the intracellular levels of threonine deaminase 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). However, the transcriptional activities of the upstream ilvpG and the internal ilvpE promoters do not increase. Therefore, the growth rate-related expression of this operon is not regulated at the level of transcription initiation. Unlike other wild-type E. coli strains, E. coli K-12 contains a polar frameshift mutation in the ilvG gene (R. P. Lawther, D. H. Calhoun, C. W. Adams, C. A. Hauser, J. Gray, and G. W. Hatfield, Proc. Natl. Acad. Sci. USA 78:922-925, 1981). In an E. coli K-12 (IlvG+) 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 frameshift mutation in the ilvG gene of this strain.