Regulatory role of microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase in a tobacco mutant that overproduces sterols.

In a tobacco mutant callus, containing up to tenfold more sterols than the wild-type genotype, HMG-CoA reductase activity is increased by a factor of approximately three, as is the case in mutant seedlings and plants. The rate of HMG-CoA synthesis from acetyl-CoA by the coupled enzyme system acetoacetyl-CoA thiolase/HMG-CoA synthase, as well as its conversion to acetyl-CoA plus acetoacetate by action of HMG-CoA lyase are not affected. These results confirm the key-regulating role of HMG-CoA reductase in sterol biosynthesis, which seems not to be confined only to the animal kingdom, but can also be extended to plants.