Deciphering the function of an ORF: Salmonella enterica DeoM protein is a new mutarotase specific for deoxyribose

We identified in Salmonella enterica serovar Typhi a cluster of four genes encoding a deoxyribokinase (DeoK), a putative permease (DeoP), a repressor (DeoQ), and an open reading frame encoding a 337 amino acid residues protein of unknown function. We show that the latter protein, called DeoM, is a hexamer whose synthesis is increased by a factor over 5 after induction with deoxyribose. The CD spectrum of the purified recombinant protein indicated a dominant contribution of betatype secondary structure and a small content of alpha-helix. Temperature and guanidinium hydrochloride induced denaturation of DeoM indicated that the hexamer dissociation and monomer unfolding are coupled processes. DeoM exhibits 12.5% and 15% sequence identity with galactose mutarotase from Lactococcus lactis and respectively Escherichia coli, which suggested that these three proteins share similar functions. Polarimetric experiments demonstrated that DeoM is a mutarotase with high specificity for deoxyribose. Site-directed mutagenesis of His183 in DeoM, corresponding to a catalytically active residue in GalM, yielded an almost inactive deoxyribose mutarotase. DeoM was crystallized and diffraction data collected for two crystal systems, confirmed its hexameric state. The possible role of the protein and of the entire gene cluster is discussed in connection with the energy metabolism of S. enterica under particular growth conditions.