The predominant membrane lipid in Bacillus megaterium ATCC 14581, phosphatidylglycerol (PG), is present in two distinct pools, as shown by [3P]phosphate incorporation and chase experiments. One pool (PGJ) undergoes rapid turnover of

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Pulse-chase experiments in Bacillus megaterium ATCC 14581 with [U-14C]palnitate, L-[U-_4C]serine, and [U-'4C]glycerol showed that a large pool of phosphatidylglycerol (PG) which exhibited rapid turnover in the phosphate moiety (PGJ) underwent very rapid interconversion with the large diglyceride (DG) pool. Kinetics of DG labeling indicated that the fatty acyl and diacylated glycerol moieties of PGt were also utilized as precursors for net DG formation. The [U-'4C]glycerol pulse-chase results also confirmed the presence of a second, metabolically stable pool of PG (PG.), which was deduced from [32P]phosphate studies. The other major phospholipid, phosphatidylethanolamine (PE), exhibited pronounced lags relative to PG and DG in "C-fatty acid, [14C]glycerol, and [P]phosphate incorporation, but not for incorporation of L-[U-'4C]serine into the ethanolamine group of PE or into the serine moiety of the small phosphatidylserine (PS) pool. Furthermore, initial rates of L-[U-'4C]serine incorporation into the serine and ethanolamine moieties of PS and PE were unaffected by cerulenin. The results provided compelling in vivo evidence that de novo PGt, PS, and PE syntheses in this organism proceed for the most part sequentially in the order PGt-+ PS-+ PE rather than via branching pathways from a common intermediate and that the phosphatidyl moiety in PS and PE is derived largely from the corresponding moiety in PGt, whereas the DG pool indirectly provides an additional source for this conversion by way of the facile PGt ± DG interconversion.

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