The hisB gene, found in the enterobacteria (such as E. coli), in Campylobacter jejuni and in Xylella/Xanthomonas encodes a protein involved in catalysis of two step in histidine biosynthesis (the sixth and eight step), namely the bifunctional Imidazoleglycerol-phosphate dehydratase/histidinol-phosphatase.
The HIS3 gene, found in the Saccharomyces cerevisiae yeast, encodes a protein called Imidazoleglycerol-phosphate dehydratase which catalyses the sixth step in histidine biosynthesis.