Regulation of the synthesis of E. coli elongation factor Tu

Rapidly growing E. coli with two active genes (tufA, tufB) for elongation factor (EF) Tu contains three times as much EF-Tu (tuf) mRNA as EF-G (fus) mRNA on a molar basis, but about seven times as much EF-Tu as EF-G or ribosomes. The concentration and translational efficiency of fus (EF-G) mRNA is about that for ribosomal protein mRNAs. The high molar concentration of EF-Tu relative to EF-G or ribosomes is achieved in part by translating tuf mRNA more efficiently than these other mRNAs. In a tufA+ tufB-: : Mu strain, the tuf:fus mRNA ratio is 1, but the concentration of EF-Tu and tuf mRNA is the same as in the wild-type strain. Thus cells with only an active tufA gene increase the concentration but not the translational efficiency of tuf mRNA. In such cells the concentration of fus mRNA is almost three times that in the wild-type strain. Because the tufA gene is distal to but cotranscribed with the fus gene as part of the four gene str operon, the wild-type concentration of tuf mRNA in these tufB- cells must be produced by increasing the concentration of transcript corresponding to the entire str operon. Thus transcription of the tufA gene can only proceed from the str promoter. Extracts of the tufB- cells contain tuf transcripts that correspond not only in size to the entire 4.5 kb str operon, but also to the size (approximately 1 kb) of a tuf gene. Our evidence suggests that this 1 kb tuf transcript is derived by posttranscriptional modification of the primary str operon transcript and that this modification could in part explain the high translational efficiency of tuf mRNA.