title: How many initiator tRNA genes does Escherichia coli need?
abstract: Multiple copies of a gene require enhanced investment on the part of the cell and, as such, call for an explanation. The observation that Escherichia coli has four copies of initiator tRNA (tRNAi) genes, encoding a special tRNA (tRNA(fMet)) required to start protein synthesis, is puzzling particularly because the cell appears to be unaffected by the removal of one copy. However, the fitness of an organism has both absolute and relative connotations. Thus, we carried out growth competition experiments between E. coli strains that differ in the number of tRNAi genes they contain. This has enabled us to uncover an unexpected link between the number of tRNAi genes and protein synthesis, nutritional status, and fitness. Wild-type strains with the canonical four tRNAi genes are favored in nutrient-rich environments, and those carrying fewer are favored in nutrient-poor environments. Auxotrophs behave as if they have a nutritionally poor internal environment. A heuristic model that links tRNAi gene copy number, genetic stress, and growth rate accounts for the findings. Our observations provide strong evidence that natural selection can work through seemingly minor quantitative variations in gene copy number and thereby impact organismal fitness.
authors.affiliation: Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India.
authors.forename: Laasya
authors.initials: L
authors.lastname: Samhita
authors.affiliation: Department of Molecular Reproduction, Development and Genetics, Indian Institute of Science, Bangalore, India Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India.
authors.forename: Vidyanand
authors.initials: V
authors.lastname: Nanjundiah
authors.affiliation: Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India varshney@mcbl.iisc.ernet.in.
authors.forename: Umesh
authors.initials: U
authors.lastname: Varshney
mesh_terms: D050260:Carbohydrate Metabolism; D003198:Computer Simulation; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology*; D008954:Models, Biological; D009154:Mutation; D012329:RNA, Bacterial / Q000235:genetics / Q000378:metabolism*; D012343:RNA, Transfer / Q000235:genetics / Q000378:metabolism*
chemical_list: D012329:RNA, Bacterial; D012343:RNA, Transfer