A process for producing purine nucleosides which comprises breeding and culturing microorganisms having a gene encoding an enzyme wherein the feedback inhibition has been substantially relieved by substituting one or two amino acid residues in PRPP amidotransferase encoded by Escherichia coli purF, a gene encoding a protein wherein the repressor in the purine nucleotide biosynthesis system encoded by purR has been inactivated and a gene encoding an enzyme wherein purine nucleoside phosphorylase encoded by deoD has been inactivated, a gene encoding an enzyme wherein succinyl-AMP synthase encoded by purA has been inactivated, a gene encoding an enzyme wherein 6-phosphogluconate dehydratase encoded by edd has been inactivated and a gene encoding an enzyme wherein phosphoglucose isomerase encoded by pgi has been inactivated, or the like.
