Growth rate regulation of translation initiation factor IF3 biosynthesis in Escherichia coli

Abstract

infC, the gene encoding translation initiation factor IF3 in Escherichia coli, can be transcribed from three promoters. Two of these promoters, P(I1) and P(I2), are located in the upstream thrS sequence which codes for threonyl-tRNA synthetase. Previous studies had shown that P(I2) was the major promoter for infC. In the present study, the extent of transcription from P(I1) and/or P(I2) at a variety of steady-state growth rates was analyzed by promoter fusion studies. P(I2) was the more active promoter (two- to threefold stronger than P(I1)) at all growth rates tested. A fusion plasmid containing both P(I1) and P(I2) exhibited a transcription level approximately equal to the sum of those observed with the fusion plasmids containing the individual promoters. The transcriptional activities of P(I1) and P(I2) did not change as the growth rate was varied from 0.3 to 1.7 doublings per h. In contrast, a fusion plasmid carrying the rrnB P1 promoter displayed the expected growth rate response. The steady-state concentrations of infC mRNA in cells grown at different rates were measured and found not to vary. These results indicate that the previously reported growth rate regulation of IF3 biosynthesis neither is accomplished by transcriptional control nor is a result of differential mRNA stability. In view of these results, the steady-state levels of IF3 in cells grown at a number of different growth rates were determined by quantitative immunoblotting. IF3 levels were found to vary with growth rate in a manner essentially identical to that observed for ribosomes. A model accounting for these results and describing a mechanism for coordinate growth rate-regulated expression of ribosomes and IF3 is presented.