O-nitrophenyl galactoside, an unnatural substrate for beta galactosidase. It is cleaved by this enzyme (see illustration) into galactose and o-nitrophenol (a yellow compound, easily assayed spectrophotometrically). ONPG has been extensively used to determine enzyme activity associated with mutants of the lac operon (q.v.) in E. coli. Unlike IPTG (q.v.), ONPG is not an inducer of the operon, so these two substances are often used in combination.
Subjects: Genetics and Genomics.