Our laboratory has previously reported a structurally and mechanistically related family of beta-hydroxyacid dehydrogenases with significant homology to beta-hydroxyisobutyrate dehydrogenase. A large number of the members of this family are hypothetical proteins of bacterial origin with unknown identity in terms of their substrate specificities and metabolic roles. The Escherichia coli beta-hydroxyacid dehydrogenase homologue corresponding to the locus was cloned and expressed with a 6-histidine tag for specific purification. The purified recombinant protein very specifically catalyzed the NAD(+)-dependent oxidation of d-glycerate and the NADH-dependent reduction of tartronate semialdehyde, identifying this protein as a tartronate semialdehyde reductase. Further evidence for identification as tartronate semialdehyde reductase is the observation that the coding region for this protein is directly preceded by genes coding for hydroxypyruvate isomerase and glyoxylate carboligase, two enzymes that synthesize tartronate semialdehyde, producing an operon clearly designed for d-glycerate biosynthesis from tartronate semialdehyde. The single beta-hydroxyacid dehydrogenase homologue from Haemophilus influenzae was also cloned, expressed, and purified with a 6-histidine tag. This protein also catalyzed the NAD(+)-dependent oxidation of d-glycerate but was significantly more efficient in the oxidation of four-carbon beta-hydroxyacids like d-hydroxybutyrate and d-threonine. This enzyme differs from all the presently known beta-hydroxybutyrate dehydrogenases which are well established members of the short chain dehydrogenase/reductase superfamily.