Chapter 3: Protein Production in Bacteria & Yeast

Protein Production in Bacteria & Yeast begins by contrasting traditional protein extraction from animal tissues with recombinant DNA technology, which offers cost-effective, pure, and virus-free pharmaceutical products like hormones and enzymes. The text details the use of Escherichia coli as the primary bacterial host due to its rapid growth and well-understood genetics. Key methods for introducing DNA into bacteria are explained, including transformation (using calcium chloride or electroporation), conjugation (plasmid mobilization), and transduction (bacteriophage injection). The chapter provides an in-depth analysis of cloning vectors, differentiating between general-purpose plasmids like pBR322 and pUC series, which utilize antibiotic resistance and blue-white screening (alpha-complementation) for clone identification, and specialized vectors like bacteriophage lambda, cosmids, and Bacterial Artificial Chromosomes (BACs) designed for carrying larger genomic DNA fragments. Strategies for shotgun cloning, cDNA synthesis using reverse transcriptase to bypass introns, and the Polymerase Chain Reaction (PCR) for amplifying specific DNA sequences are thoroughly examined. The discussion shifts to maximizing protein expression using strong, regulatable promoters (such as lac, trp, and T7), optimizing ribosome-binding sites (Shine-Dalgarno sequence), and addressing codon usage bias. Challenges such as the formation of insoluble inclusion bodies are addressed, with solutions ranging from using fusion proteins (like GST or protein A tags) and molecular chaperones to facilitate refolding, to employing secretion vectors that target the periplasmic space. The chapter then transitions to yeast systems, specifically Saccharomyces cerevisiae, highlighting their ability to perform eukaryotic post-translational modifications like glycosylation and disulfide bond formation. Various yeast vectors—Integrative (YIp), Replicating (YRp), Episomal (YEp), Centromeric (YCp), and Yeast Artificial Chromosomes (YAC)—are categorized based on stability and copy number. Finally, the text reviews industrial applications, including the production of chymosin (rennin) for cheese making and the Hepatitis B surface antigen vaccine, illustrating the optimization of secretion signals (like the alpha-mating factor) and the use of non-conventional yeasts like Pichia pastoris and Kluyveromyces lactis for high-yield secretion.