[Sense codon in Escherichia coli are translated in context].

The nucleotide frequencies 5' and 3' to the sense codons in highly and weakly expressed genes have been investigated by the chi-squares method. A comparison between the experimental and computer-generated random nucleotide sequences (in which each codon is substituted by a random synonymous one) was made. It was shown that the choice of a particular codon among the synonymous ones in a given position of the gene depends on the three nucleotides 3' and 5' adjacent to the codon in highly expressed genes (the triplet 3' and a single nucleotide 5' to the codons in weakly expressed genes). Concrete patterns for the preferable choice of synonymous codons depending on their contexts are presented. It is suggested that these constraints are related to the efficiency of messenger translation. The constraints on the amino acid sequences of encoded proteins also lead to statistically significant bases in nucleotide frequencies around the sense codons. The biological role of these constraints is discussed.