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

The sulfolipid 6-sulfo-alpha-D-quinovosyl diacylglycerol is found in the photosynthetic membranes of all plants and most photosynthetic bacteria. Progress toward the elucidation of the pathway for sulfolipid biosynthesis has been slow in the past. However, the recent isolation of three genes of the photosynthetic bacterium Rhodobacter sphaeroides known 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 nucleotide-dependent glycosyltransferases. The UDP-sulfoquinovose:diacylglycerol 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 sulfoquinovosyltransferase of R. sphaeroides, it was inactivated by gene replacement avoiding polar mutagenesis. The resulting sulfolipid-deficient mutant defines a new gene, designated sqdD. Mutant cells grown in the presence of [S-35]sulfate accumulate a water-soluble S-35-labeled compound. The purified compound was tentatively identified by co-chromatography with standards and enzymatic conversion as UDP-sulfoquinovose, the final precursor of sulfolipid biosynthesis. This result strongly suggests that the inactivation of sqdD causes a metabolic block in the last step of sulfolipid biosynthesis.