ACCUMULATION OF UDP-SULFOQUINOVOSE IN A SULFOLIPID-DEFICIENT MUTANT OF RHODOBACTER-SPHAEROIDES

The sulfolipid 6-sulfo-alpha-D-quinovosyl diacylglycerol is found in t he photosynthetic membranes of all plants and most photosynthetic bact eria. Progress toward the elucidation of the pathway for sulfolipid bi osynthesis has been slow in the past. However, the recent isolation of three genes of the photosynthetic bacterium Rhodobacter sphaeroides k nown to be involved in sulfo-lipid biosynthesis provides promising new opportunities. Two of the genes flank an open reading frame predicted to encode a protein with amino acid sequence similarity to sugar nucl eotide-dependent glycosyltransferases. The UDP-sulfoquinovose:diacylgl ycerol sulfoquinovosyltransferase thought to catalyze the last step of sulfolipid biosynthesis belongs to this group of glycosyltransferases . To test whether this open reading frame encodes the sulfoquinovosylt ransferase of R. sphaeroides, it was inactivated by gene replacement a voiding polar mutagenesis. The resulting sulfolipid-deficient mutant d efines a new gene, designated sqdD. Mutant cells grown in the presence of [S-35]sulfate accumulate a water-soluble S-35-labeled compound. Th e purified compound was tentatively identified by co-chromatography wi th standards and enzymatic conversion as UDP-sulfoquinovose, the final precursor of sulfolipid biosynthesis. This result strongly suggests t hat the inactivation of sqdD causes a metabolic block in the last step of sulfolipid biosynthesis.