Accumulation of Sulfoquinovosyl-1-O-dihydroxyacetone in a Sulfolipid-Deficient Mutant ofRhodobacter sphaeroidesInactivated insqdC

The biosynthesis of the sulfolipid sulfoquinovosyl diacylglycerol in the purple bacteriumRhodobacter sphaeroidesrequires at least four genes:sqdA, sqdB, sqdC,andsqdD.As part of our strategy aimed at the elucidation of the function of the differentsqdgene products, we insertionally inactivatedsqdCofR. sphaeroides.The resultingsqdCnull mutant showed only a 90% reduction in sulfolipid content. Apparently, thesqdCgene product is required for optimal sulfolipid biosynthesis, but either catalyzes no essential reaction in the pathway or can be functionally replaced to a certain extent by a different protein. The mutant accumulated a35S-labeled compound that was purified to homogeneity from cell extracts. Matrix-assisted laser desorption mass spectrometry and nuclear magnetic resonance spectroscopy provided conclusive structural evidence to identify the compound as α-d-sulfoquinovosyl-1-O-dihydroxyacetone that exists in two interconvertible, keto and hemiacetal forms. Incubation of wild-type protein extracts with the labeled compound did not result in the incorporation into sulfolipid as would be expected for an intermediate of the pathway. Based on our results we propose that thesqdCgene product mediates the substrate specificity of the UDP-sulfoquinovose:diacylglycerol sulfoquinovosyltransferase that is encoded bysqdDand that catalyzes the final reaction of sulfolipid biosynthesis.