Growth rate dependence of transfer RNA abundance in Escherichia coli.

We have tested the predictions of a model that accounts for the codon preferences of bacteria in terms of a growth maximization strategy. According to this model the tRNA species cognate to minor and major codons should be regulated differently under different growth conditions: the isoacceptors cognate to major codons should increase at fast growth rates while those cognate to minor codons should decrease at fast growth rates. We have used a quantitative Northern blotting technique to measure the abundance of the methionine and the leucine isoacceptor families over growth rates ranging from 0.5 to 2.1 doublings per hour. Five tRNA species that are cognate to major codons (tRNA(eMet), tRNA(1fMet), tRNA(2fMet), tRNA(1Leu) and tRNA(3Leu) increase both as a relative fraction of total tRNA and in absolute concentration with increasing growth rates. Three tRNA species that are cognate to minor codons (tRNA(2Leu), tRNA(4Leu) and tRNA(5Leu) decrease as a relative fraction of total RNA and in absolute concentration with increasing growth rates. These data suggest that the abundances of groups of tRNA species are regulated in different ways, and that they are not regulated simply according to isoacceptor specificity. In particular, the data support the growth optimization model for codon bias.