Coordinate regulation of unsaturated phospholipid, RNA, and protein synthesis in Mycoplasma capricolum by cholesterol.

The effect of cholesterol, epicoprostanol, and phosphatidylcholine on phospholipid, RNA, and protein synthesis was investigated in the sterol auxotroph Mycoplasma capricolum. Cells growing poorly on lanosterol were stimulated to grow more rapidly by supplementing the medium with either 2 micrograms of cholesterol or 2.2 micrograms of egg phosphatidylcholine per ml. In such cells cholesterol caused a sequential stimulation of phospholipid, RNA, and protein synthesis. Enhanced oleate incorporation into phospholipid occurred early; the rates of RNA and protein synthesis increased later. In cells supplemented with phosphatidylcholine only RNA and protein syntheses were enhanced. The addition of 2 micrograms of epicoprostanol per ml to cells growing on lanosterol promptly inhibited the rate of unsaturated phospholipid synthesis and subsequently the rate of growth. Inhibition of both processes was relieved by supplying 2 micrograms of cholesterol or 2.2 micrograms of phosphatidylcholine per ml along with the inhibitory sterol. The results suggest that cholesterol in small amounts exerts a positive regulatory effect and epicoprostanol exerts a negative one on unsaturated phospholipid synthesis and, in turn, that RNA and protein synthesis are coordinately controlled with phospholipid synthesis. The previously reported phenomenon of sterol synergism and the postulated novel role of sterols in membranes.