The effect of a phosphoribosylpyrophosphate (PRPP) synthetase gene (Prs) that was desensitized to feedback inhibition by ADP on inosine accumulation was investigated using an inosine-producing mutant of Escherichia coli. At the same time, various types of plasmid having a PRPP amidotransferase gene (purF) that was desensitized to feedback inhibition by AMP and GMP were also investigated to improve inosine productivity using a compatible plasmid containing prs with a plasmid containing purF. The recombinant E. coli I-9 harboring a low-copy-number plasmid having the desensitized-purF (pMWKQ) accumulated 3.6 g/l inosine from 40 g/l glucose in a 2-d culture. Furthermore, desensitized-prs amplification, in addition to purF, resulted in the accumulation of 6.2 g/l inosine. Additionally, through these experiments, a spontaneous mutant with an enhanced inosine-producing ability compared with the parent strain I-9 was obtained. The spontaneous mutant I-9m harboring only pMWKQ and I-9m harboring both pMWKQ and pSTVDA (a plasmid having the desensitized-prv) accumulated 6.7 g/l and 7.5 g/l inosine, respectively, from 40 g/l glucose in a 3-d culture.