ISOLATION AND FUNCTIONAL EXPRESSION IN ESCHERICHIA-COLI OF A GENE ENCODING PHOSPHATIDYLETHANOLAMINE METHYLTRANSFERASE (EC-2.1.1.17) FROM RHODOBACTER-SPHAEROIDES

Phosphatidylcholine is a major component of membranes in most eukaryot es, but it is found only in a small number of bacteria, where it is sy nthesized by N-methylation of phosphatidylethanolamine. In yeast and o ther fungi the methylation of phosphatidylethanolamine to phosphatidyl choline proceeds in two steps: the methylation of phosphatidylethanola mine by phosphatidylethanolamine methyltransferase followed by the met hylation of monomethylphosphatidylethanolamine by phospholipid methylt ransferase. Here we describe the isolation of two allelic phosphatidyl choline-deficient mutants of Rhodobacter sphaeroides which are unable to methylate phosphatidylethanolamine, monomethylphosphatidylethanolam ine, or dimethylphosphatidylethanolamine. A DNA fragment containing a gene designated pmtA, which encodes a 22.9-kDa protein, was found to c omplement both mutants. Expression of this gene in Escherichia coli, w hich normally lacks phosphatidylcholine or methylated derivatives of p hosphatidylethanolamine, resulted in the formation of phosphatidylchol ine. A protein extract derived from the E. coli strain expressing the pmtA gene was able to convert phosphatidylethanolamine, mono- and dime thylphosphatidylethanolamine into phosphatidylcholine. Based on these data we conclude that the product of the pmtA gene catalyzes a sequenc e of three chemically distinct, methylation reactions beginning with p hosphatidylethanolamine and leading to the formation of phosphatidylch oline in R. sphaeroides.