RESUMEN
A yeast expression plasmid was constructed containing a cardenolide biosynthetic module, referred to as CARD II, using the AssemblX toolkit, which enables the assembly of large DNA constructs. The genes cloned into the vector were (a) a Δ5 -3ß-hydroxysteroid dehydrogenase gene from Digitalis lanata, (b) a steroid Δ5 -isomerase gene from Comamonas testosteronii, (c) a mutated steroid-5ß-reductase gene from Arabidopsis thaliana, and (d) a steroid 21-hydroxylase gene from Mus musculus. A second plasmid bearing an ADR/ADX fusion gene from Bos taurus was also constructed. A Saccharomyces cerevisiae strain bearing these two plasmids was generated. This strain, termed "CARD II yeast", was capable of producing 5ß-pregnane-3ß,21-diol-20-one, a central intermediate in 5ß-cardenolide biosynthesis, starting from pregnenolone which was added to the culture medium. Using this approach, five consecutive steps in cardenolide biosynthesis were realized in baker's yeast.