초록
<P>An open reading frame CC1225 from the <I>Caulobacter crescentus</I> CB15 genome sequence belongs to the Gfo/Idh/MocA protein family and has 47 % amino acid sequence identity with the glucose-fructose oxidoreductase from <I>Zymomonas mobilis</I> (<I>Zm</I> GFOR). We expressed the ORF CC1225 in the yeast <I>Saccharomyces cerevisiae</I> and used a yeast strain expressing the gene coding for <I>Zm</I> GFOR as a reference. Cell extracts of strains overexpressing CC1225 (renamed as <I>Cc aaor</I>) showed some <I>Zm</I> GFOR type of activity, producing D-gluconate and D-sorbitol when a mixture of D-glucose and D-fructose was used as substrate. However, the activity in <I>Cc aaor</I> expressing strain was >100-fold lower compared to strains expressing <I>Zm gfor</I>. Interestingly, <I>C. crescentus</I> AAOR was clearly more efficient than the <I>Zm</I> GFOR in converting in vitro a single sugar substrate D-xylose (10 mM) to xylitol without an added cofactor, whereas this type of activity was very low with <I>Zm</I> GFOR. Furthermore, when cultured in the presence of D-xylose, the <I>S. cerevisiae</I> strain expressing <I>Cc aaor</I> produced nearly equal concentrations of D-xylonate and xylitol (12.5 g D-xylonate l<SUP>−1</SUP> and 11.5 g D-xylitol l<SUP>−1</SUP> from 26 g D-xylose l<SUP>−1</SUP>), whereas the control strain and strain expressing <I>Zm gfor</I> produced only D-xylitol (5 g l<SUP>−1</SUP>). Deletion of the gene encoding the major aldose reductase, Gre3p, did not affect xylitol production in the strain expressing <I>Cc aaor</I>, but decreased xylitol production in the strain expressing <I>Zm gfor</I>. In addition, expression of <I>Cc aaor</I> together with the D-xylonolactone lactonase encoding the gene <I>xylC</I> from <I>C. crescentus</I> slightly increased the final concentration and initial volumetric production rate of both D-xylonate and D-xylitol. These results suggest that <I>C. crescentus</I> AAOR is a novel type of oxidoreductase able to convert the single aldose substrate D-xylose to both its oxidized and reduced product.</P>