High-level expression from one particular heterologous gene in Escherichia coli generally requires the optimization of codon usage. Genes encoding for Hepatitis C virus core protein (HCcAg), human interferon alpha2 and 8 subtypes ((Hu)IFNalpha2 and (Hu)IFNalpha8) show a high content of AGA/AGG codons. These are encoded by the product of the dnaY gene in E coli. The proteins used in this work have a high therapeutic value and were used as models for studying the effects of these rare codons on the efficiency of heterologous gene expression in E coli. Expression plasmids were constructed to express any of these proteins and the dna Y gene product simultaneously in E. coli. After dna Y gene expression, HCcAg, and (Hu)IFNa2 expression levels increased 5 and 3 times, respectively. However, (Hu)IFNalpha8 expression was barely detected either supplying or not the additional dna Y gene product. These results suggest that the high frequency of AGA/AGG codons present in the HCcAg and (Hu)IFNalpha2 genes could be one of the factors limiting its expression in E coli. Nevertheless, for (HU)IFNalpha8 it seems that other factors prevail upon the lack of dnaY product. Data presented here for HCcAg and (HU)IFNalpha2 expressions proved the value of this approach to obtain therapeutic proteins in E coli. (C) 2002 Elsevier Science (USA). All rights reserved.