Process for preparing genetically transformed yeasts capable of producing a molecule of interest at a high titre

A method for producing hydrocortisone from yeast to achieve production of at least 100 mg/L hydrocortisone, the method comprising:

• (a) providing two integration plasmids, each integration plasmid comprising at least two expression cassettes, such that the two integration plasmids together comprise at least four expression cassettes, wherein the four expression cassettes of the two integration plasmids are P450scc, adrenodoxin (ADX), P450c11, and 3β-hydroxysteroid dehydrogenase (3β-HSD);
• (b) stably integrating multiple copies of the two integration plasmids into a population of yeast cells, by co-transforming the plasmids into the yeast;
• (c) performing a primary screen of 500 to 1000 transformed yeast clones, wherein selection of the yeast clones is based on the presence of the expression cassettes, or selection for the expression of a selectable marker when such a marker is present;
• (d) performing a functional secondary screen on at least 30 yeast clones selected in the primary screen to identify a yeast isolate producing at least 100 mg/L hydrocortisone, wherein the yeast isolate comprises multiple copies of each of the integration plasmids and the multiple copies of the integration plasmids are stably integrated into the chromosomes of the yeast isolate;
• (e) culturing the yeast isolate in a culture medium;
• and
• (f) recovering and purifying said hydrocortisone from the culture medium.