Candida albicans SOU1 encodes a sorbose reductase required for L‐sorbose utilization

Abstract

Previous work in our laboratory showed that L-sorbose utilization in Candida albicans is subject to a novel form of regulation which involves a reversible increase or decrease in the copy number of chromosome 5. Furthermore, the structural gene SOU1 is required for L-sorbose utilization and encodes a member of the short chain dehydrogenase family. However, the precise function of SOU1 was not known and neither was the pathway for L-sorbose utilization. We have now expressed SOU1 at a high level from a replicative plasmid having a constitutive ADH1 promoter and purified a version of Sou1p tagged with the FLAG epitope at the N-terminus. Sou1FLAGNp has a sorbose reductase activity which utilizes NADPH as a co-factor and converts L-sorbose to D-sorbitol. It can also less efficiently utilize fructose as a substrate with NADPH as a co-factor, converting fructose to mannitol. In agreement with prediction, the purified enzyme has a subunit molecular weight of 31 kDa and a pI of about 4.8. It probably consists of four identical subunits and has a pH optimum of 6.2. The L-sorbose utilization pathway in C. albicans probably converts L-sorbose to fructose-6-phosphate via D-sorbitol as an intermediate. The first step is catalysed by Sou1p. We also found that C. albicans extracts have a D-sorbitol-6-phosphate dehydrogenase activity, not encoded by SOU1, which utilizes NADP as a co-factor. This activity has not been described previously in yeasts and may be involved in the conversion of phosphorylated D-sorbitol to fructose-6-phosphate or glucose-6-phosphate.

Chromosome 5 Monosomy of Candida albicans Controls Susceptibility to Various Toxic Agents, Including Major Antifungals

“…The regions are scattered along Ch5, and the final number of regions is yet to be established. The monosomic condition of Ch5 downregulates, as expected, at least CSU51 (orf19.1105.2) and CSU53 (orf19.3931) from Ch5 and also upregulates SOU1 (sorbose utilization) from Ch4 that encodes sorbose reductase, which catalyzes the first step in the catabolic pathway of L-sorbose (11,13,14, and E. Rustchenko, unpublished data). Furthermore, antisense regulation of CSUs is involved, as at least CSU51 and CSU53 produce, in addition to sense transcripts, long noncoding antisense transcripts, designated ASUs (activation of sorbose utilization).…”

l -Sorbose Reductase and Its Transcriptional Regulator Involved in l -Sorbose Utilization of Gluconobacter frateurii

“…A major part of the substrate is oxidized by membranebound PQQ-GLDH and FAD-SLDH, and then L-sorbose accumulates in the culture medium. After L-sorbose accumulates to some extent and D-sorbitol is exhausted, the cells start to utilize L-sorbose, probably similar to the case in C. albicans (8), through fructose 6-phosphate. During the accumulation of Lsorbose, SboR is expressed together with SboA and may compete with the unknown activator by binding to the same operator site for sboRA transcription; however, it probably does not repress its expression because of the presence of a derepressing molecule.…”

“…It has been reported that L-sorbose is incorporated into cells of Agrobacterium tumefaciens, Neurospora crassa, Aspergillus nidulans, and Candida albicans without phosphorylation (8). In contrast, in Escherichia coli, Lactobacillus casei, and Klebsiella pneumoniae, L-sorbose is phosphorylated to L-sorbose-1-phosphate at the cell surface during transport into the cell and reduced to D-sorbitol-6-phosphate by L-sorbose-1-phosphate reductase (26,27,30).…”

“…In contrast, in Escherichia coli, Lactobacillus casei, and Klebsiella pneumoniae, L-sorbose is phosphorylated to L-sorbose-1-phosphate at the cell surface during transport into the cell and reduced to D-sorbitol-6-phosphate by L-sorbose-1-phosphate reductase (26,27,30). In either case, an NAD(P)H-dependent reductase catalyzing the reduction of L-sorbose or L-sorbose-1-phosphate is essential for L-sorbose utilization in these microorganisms (8). The metabolic pathways of D-sorbitol, Lsorbose, and their metabolites in Gluconobacter strains have been studied previously (9,20,22), and these C sources were shown to be utilized through a nonphosphorylating pathway; however, no transcriptional analysis has yet been performed.…”

Aneuploid Chromosomes Are Highly Unstable during DNA Transformation of Candida albicans

“…They found that loss of chromosome 5 (Chr5) permitted growth on sorbose (53) and that subsequent reduplication of Chr5 under nonselective conditions reversed this phenotype. This phenotype is apparently due to multiple genes on the right arm of Chr5 that negatively regulate SOU1 (sorbose utilization 1), which is found on Chr4 (44,54).…”