In steroidogenic animal tissues cytochrome P450SCCcatalizes the conversion of cholesterol into pregnenolone, a common metabolic precursor of all steroid hormones. To study the possibility of functioning of mammalian cytochrome P450SCCin plants and the mechanism of its integration in the plant steroidogenic system, transgenic plants of tobacco Nicotiana tabacum L. were developed carrying cDNA of CYP11A1 encoding cytochrome P450SCCof bovine adrenal cortex. Pregnenolone, a product of the reaction catalyzed by cytochrome P450SCC, was discovered in the steroid-containing fraction of transgenic plants. Transgenic plants are characterized by a reduced period of vegetative development (early flowering and maturation of bolls) and increased productivity. The contents of soluble protein and carbohydrates in leaves and seeds of transgenic plants are essentially higher than the contents of these components in leaves and seeds of control plants. © Pleiades Publishing, Inc., 2009.