Control of RNA and protein synthesis by the concentration of Trp-tRNATrp in Escherichia coli infected with bacteriophage MS2.

The effect of varying the concentration of Trp-tRNATrp in Escherichia coli infected with bacteriophage MS2 has been studied by varying the amount of exogenously added tryptophan (Trp) to cells bearing a mutation which results in a tryptophanyl-tRNA synthetase with a higher Km value for Trp. The phenotype of the mutant has been confirmed by measuring the level of tRNATrp which can be aminoacylated in vivo, and the mutant has also been shown to have elevated tRNATrp levels compared to wild-type. The growth of MS2 decreases continuously with decreasing Trp concentration (and hence, decreasing Trp-tRNATrp concentration). This appears to be due to reduced gene expression, since at later times in infection the amount of MS2 coat protein synthesized likewise decreases continuously with decreasing Trp concentration. However, there is little decrease in the amount of coat protein or replicase synthesized during the first few minutes after the Trp concentration shift. A continuous increase in the average polysome size distribution is seen as the Trp concentration is decreased. MS2 RNA synthesis also decreases continuously with decreasing Trp concentration, and is shut off in the absence of Trp. This does not seem to be due to ppGpp as these cells are functionally relaxed under these conditions, nor does it seem to be due to degradation of pre-existing template. Addition of chloramphenicol abolishes the effect of Trp concentration on RNA synthesis. The data are consistent with a model in which ribosomes pause at Trp codons in the absence of Trp-tRNATrp, while other ribosomes queue behind and continue to load onto the message. The reduction of RNA synthesis would then be a consequence of coupling to translation.