Abstract
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 sulfolipid biosynthesis provides promising new opportunities. Two of the genes flank an open reading Rhodobacter sphaeroides known to be involved in sulfolipid 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:diacylglcerol 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 [35S]sulfate accumulate a water-soluble 35S-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.
Highlights
The sulfolipid sulfoquinovosyl diacylglycerol is found in all plants and most photosynthetic bacteria [1, 2]
Since the effect was specific for UDP-sulfoquinovose as compared to other sulfosugar nucleotides, it was concluded that in spinach chloroplasts a UDPsulfoquinovose:diacylglycerol sulfoquinovosyltransferase catalyzes the last step of sulfolipid biosynthesis
We set out to identify UDPsulfoquinovose in the mutant cells which we expected to accumulate due to a metabolic block in the final step of sulfolipid biosynthesis, if the protein encoded by the open reading frame would be involved in catalyzing this final reaction
Summary
The sulfolipid sulfoquinovosyl diacylglycerol is found in all plants and most photosynthetic bacteria [1, 2]. SqdB and sqdC, were found to flank an open reading frame predicted to encode a protein with an amino acid sequence similar to that of glycogenin [9], a sugar nucleotide-dependent glycosyltransferase, but no mutant affecting this open reading frame was available. Given this sequence similarity and the location of the open reading frame within one of the sulfolipid operons of R. sphaeroides, it is tempting to predict that the open reading frame may encode a protein catalyzing the transfer of sulfoquinovose during final assembly of the sulfolipid in analogy to the reaction in spinach chloroplasts. We set out to identify UDPsulfoquinovose in the mutant cells which we expected to accumulate due to a metabolic block in the final step of sulfolipid biosynthesis, if the protein encoded by the open reading frame would be involved in catalyzing this final reaction
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