Significance of multiple copies of initiator tRNA genes in Escherichia Coli

Abstract

E. coli has four copies of initiator tRNA genes-the highest number known in any eubacterium. Among prototrophs, strains carrying four initiator tRNA genes have a growth advantage over strains carrying three initiator tRNA genes in long term competition. In strains auxotrophic for several amino acids, having three initiator tRNA genes is advantageous over having four. This advantage is reversed only under specific conditions where the strains are maintained under nutrient rich conditions by continuous subculturing. A strain that is auxotrophic for only one amino acid shows intermediate behaviour. Strains with four initiator tRNA genes out compete isogenic strains with three under most conditions. However, when the environment is limiting for the amino acid for which the bacterium is auxotrophic, the strain carrying three initiator tRNA genes has a growth advantage. Additionally, having fewer initiator tRNA genes reduces the overall levels of protein synthesis. Natural E. coli isolates from different sources show variation in the number of initiator tRNA genes, as tested by PCR. Our results suggest that there is a dynamic shift between three and four initiator tRNA genes depending on the nutritional status of the environment. It appears that having multiple identical copies of this gene may confer adaptability to changing environments.