Role of the AGA/AGG codons, the rarest codons in global gene expression in Escherichia coli.

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Abstract
AGA and AGG codons for arginine are the least used codons in Escherichia coli. Previous findings have shown that these codons are used preferentially within the first 25 codons in E. coli genes. More than 100 genes having a single AGA/AGG codon within the first 25 codons were identified to be associated with various essential cellular functions. The lacZ gene, containing 5 AGG codons after the tenth codon from the initiation codon, was constructed as a model system. The production of beta-galactosidase was inhibited almost completely during the stationary phase, whereas the production of the control beta-galactosidase without AGG codons was not. The inhibitory effect by the 5 AGG codons was substantially suppressed either by coexpressing the argU gene for tRNA(ArgUCU/CCU) or by moving the 5 AGG codons by > 50 codons away from the initiation codon. In addition, the production of a number of proteins resolved by two-dimensional gel electrophoresis was enhanced significantly during the stationary phase in the cells harboring a plasmid containing argU. At least one of them was identified as the hns product encoded by an ORF having an AGA codon at the nineteenth position. On the basis of these results, it is proposed that the expression of a group of essential genes for various cellular functions that have a single AGA/AGG codon very close to the initiation codon are globally regulated by the availability of the least abundant tRNA(ArgUCU/CCU). A model for this regulation is proposed.