PREFERENCE FOR GUANOSINE AT FIRST CODON POSITION IN HIGHLY EXPRESSED ESCHERICHIA-COLI GENES - A RELATIONSHIP WITH TRANSLATIONAL EFFICIENCY

The variation in base composition at the three codon sites in relation to gene expressivity, the latter estimated by the Codon Adaptation In dex, has been studied in a sample of 1371 Escherichia coli genes. Corr elation and regression analyses show that increasing expression levels are accompanied by higher frequencies of base G at first, of base A a t second and of base C at third codon positions. However, correlation between expressivity and base compositional biases at each codon site was only significant and positive at first codon position. The prefere nce for G-starting codons as gene expression level increases is discus sed in terms of translational optimization.