ACCUMULATION OF SULFOQUINOVOSYL-1-O-DIHYDROXYACETONE IN A SULFOLIPID-DEFICIENT MUTANT OF RHODOBACTER-SPHAEROIDES INACTIVATED IN SQDC

The biosynthesis of the sulfolipid sulfoquinovosyl diacylglycerol in t he purple bacterium Rhodobacter sphaeroides requires at least four gen es: sqdA, sqdB, sqdC, and sqdD. As part of our strategy aimed at the e lucidation of the function of the different sqd gene products, we inse rtionally inactivated sqdC of R. sphaeroides. The resulting sqdC null mutant showed only a 90% reduction in sulfolipid content. Apparently, the sqdC gene product is required for optimal sulfolipid biosynthesis, but either catalyzes no essential reaction in the pathway or can be f unctionally replaced to a certain extent by a different protein. The m utant accumulated a S-35-labeled compound that was purified to homogen eity from cell extracts. Matrix-assisted laser desorption mass spectro metry and nuclear magnetic resonance spectroscopy provided conclusive structural evidence to identify the compound as alpha-D-sulfoquinovosy l-1-O-dihydroxyacetone that exists in two interconvertible, keto and h emiacetal forms. Incubation of wild-type protein extracts with the lab eled compound did not result in the incorporation into sulfolipid as w ould be expected for an intermediate of the pathway. Based on our resu lts we propose that the sqdC gene product mediates the substrate speci ficity of the UDP-sulfoquinovose:diacylglycerol sulfoquinovosyltransfe rase that is encoded by sqdD and that catalyzes the final reaction of sulfolipid biosynthesis. (C) 1997 Academic Press.