We have deleted the chromosomal rpsA gene, encoding ribosomal protein S1, from an Escherichia coli strain carrying a plasmid where rpsA was controlled by the lac promoter and operator. This exogenous source of protein S1 was essential for growth. Thus we have verified the absolute requirement for protein S1. To see if translation of individual mRNAs differed in the requirements for protein S1, we removed the inducer and followed the time-course of the synthesis of several individual proteins and of total RNA, DNA and protein. Growth immediately shifted from being exponential to being linear, with a rate of protein synthesis defined by the pre-existing amount of protein S1. The expression pattern of the individual proteins indicated that the translation of all mRNAs was dependent on protein S1. Unexpectedly, we found that depletion for protein S1 for extended periods introduced a starvation for amino acids. Such starvation was indicated by an increased synthesis of ppGpp and could be reversed by addition of a mixture of all 20 amino acids. Measurements of the peptide chain elongation rate in vivo showed that ribosomes without protein S1 were unable to interfere with the peptide chain elongation rate of the active ribosomes and that, therefore, protein S1 was unable to diffuse from one ribosome to another during translation. We conclude that protein S1-deficient ribosomes are totally inactive in peptide chain elongation on most, if not all, naturally occurring E. coli mRNAs.
Copyright 1998 Academic Press.