Feedback control of ribosome synthesis in Escherichia coli is dependent on eight critical amino acids

Abstract

When bacteria growing in minimal medium are supplied with exogenous amino acids, they respond by increasing the synthesis of ribosomes; this leads to more protein synthesis capacity and faster growth. To examine how amino acids control the synthesis of ribosomes, two strategies were used. First, single amino acids were added to bacteria growing in minimal medium and their effect on the relative strength of the rrnB P1 promoter was determined. The addition of any one of eight amino acids (alanine, glutamine, and glutamic acid, isoleucine, leucine, methionine, serine, valine) increased the strength of the P1 promoter by 1.25- to 2.0-fold with no appreciable effect on transcription from an isolated rrn P2 promoter or on the bacterial growth rate. The effects of adding combinations of these critical amino acids were partially additive. When any one of the other amino acids was added, no discernable stimulation in relative P1 expression or growth was observed. In the second strategy, all amino acids were present in the growth medium, but the carbon source was altered to change the growth rate. In this case the relative strength of the P1 promoter was always constant and maximal. We suggest that addition of any of the eight critical amino acids reduces the ppGpp synthesis activity of the spoT gene product; the lower ppGpp levels, in turn, increase the strength of the rrn P1 promoters. It is suggested that these amino acids are involved in a feedback chain of reactions that control the rate of ribosome function by adjusting the rate of ribosome synthesis.