A gene coding for 3-deoxy-D-manno-octulosonic-acid transferase in Escherichia coli. Identification, mapping, cloning,...

An autoradiographic assay applicable to colonies immobilized on filter paper was developed for obtaining temperature-sensitive mutants of Escherichia coli defective in the transfer of 3-deoxy-D-manno-octulosonic acid (KDO) from CMP-KDO to a tetraacyldisaccharide 1,4'-bisphosphate precursor of lipid A, designated lipid IVA. Cell-free extracts from two mutants found in a population of 30,000 mutagen-treated cells showed normal KDO transferase activity when assayed at 30 degrees C, but almost no activity at 42 degrees C. The mutation was mapped by mating one of the mutants with different Hfr strains and analyzing genetic linkage of KDO transferase activity to selectable markers. The lesion was located to a position between 80 and 84 min on the E. coli chromosome. A plasmid from the Clarke and Carbon collection (Clarke, L., and Carbon, J. (1976) Cell 9, 91-99), pLC17-24, known to contain genes from the rfa region (81 min), was shown to overexpress KDO transferase activity 4-5 times and to correct the mutation when the plasmid was conjugated into the mutant strains. The KDO transferase gene, designated kdtA, was subcloned from pLC17-24 into a multicopy vector. The resulting plasmid, pCL3, overproduced transferase activity approximately 100-fold. The kdtA gene was shown to code for a 43-kDa polypeptide, as judged by radiolabeling of minicells. Its DNA sequence was determined. The results demonstrate that overexpression of this single gene product greatly stimulates the incorporation of two stereochemically distinct KDO residues during lipopolysaccharide biosynthesis in extracts of E. coli.