How did an American Company, Eli Lilly use the knowledge of r-DNA technology to produce insulin?

The gene responsible for insulin production in humans is inserted into the plasmids of E.coli bacteria. This E.coli bacteria is then grown in larger quantities on a culture medium in the laboratory. In humans, immature pro-insulin contains A, B, and C-peptide chains. The C peptide is not present in mature insulin and is removed during maturation into insulin. The main challenge for insulin production using rDNA technique was getting insulin assembled into a mature form. In 1983, Eli Lilly, an American company, first prepared two DNA sequences corresponding to the A and B chains of human insulin and introduced them into plasmids of E. coli to produce the insulin chains. Chains A and B were produced separately, extracted, and combined by creating disulfide bonds to form human insulin (humulin).