Genetic code ambiguity confers a selective advantage on Acinetobacter baylyi

Abstract

A primitive genetic code, composed of a smaller set of amino acids, may have expanded via recursive periods of genetic code ambiguity that were followed by specificity. Here we model a step in this process by showing how genetic code ambiguity could result in an enhanced growth rate in Acinetobacter baylyi.

FIG. 1.

Growth rate advantage and enhanced misincorporation of valine due to genetic code ambiguity in A. baylyi. Growth conditions were systematically varied to determine whether certain concentrations of isoleucine and valine could result in a growth rate advantage for A. baylyi carrying the ileS_Ala mutation. (A) Growth rates of strains carrying the editing-defective and wild-type alleles of ileS, as determined at several concentrations of Ile and Val. (The shorthand “IxVy” is used to indicate the concentrations of isoleucine (x) and valine (y) in the medium; values are μM.) The following conditions were tested: limiting isoleucine and limiting valine (I30V50), dramatically increased valine (I30V500), and further addition of isoleucine to the medium (I70V500). The medium was otherwise MSglc plus 50 μM Leu. At the slower growth rate, the doubling time is ∼3.3 h, while the doubling time at the faster growth rate is ∼2.3 h. Data are based on at least three independent assays for growth rates of each of two clones, each done in triplicate. (B) At each of these conditions, the total cellular complement of macromolecules was isolated and subjected to amino acid analysis. Fraction Val, pmol Val/(pmol Ile + pmol Val). In order to isolate sufficient material, cultures were grown in flasks rather than in microplates; therefore, the data are not directly from cultures in which growth rates were determined (A). These data were acquired at the Center for Protein Sciences at the Scripps Research Institute and were qualitatively replicated at the Protein Microanalysis Facility at the University of Texas at Austin.