Influence of the codon following the initiation codon on the expression of the lacZ gene in Saccharomyces cerevisiae

A set of 32 different codons were introduced in a lacZ experssion vector (pPTK400) immediately 3′ from the AUG initiation codon. Expression of the lacZ gene was determined in Saccharomyces cerevisiae by measuring the amount of β‐galactosidase fusion protein using immuno‐gel electrophoresis. A 5·3‐fold difference in expression was found among the various constructs. It was found that there was no preference for a certain nucleotide in any position of the second codon and there was no distinct correlation between the level of tRNA corresponding to any particular second codon and expression. No correlation could be found between the local secondary structure and expression. When the overall codon usage in yeast and the codon usage in the second position of the mRNA is compared, there is no obvious significant difference in preference. This indicates that in yeast, in contrast to Escherichia coli, the codon choice at the beginning of the mRNA does not deviate from the one further downstream and is determined by the requirements for optimal translation elongation. Important determinatnts of the optimal context for an initiation codon in yeast therfore must be located mainly 5′ from this codon.