Chapter 17: Microbial DNA Technology & Genetic Engineering

Microbial DNA Technology & Genetic Engineering from Prescott's Microbiology 11th Edition outlines the foundational techniques and key discoveries that define the field of microbial DNA technologies, often exemplified by complex projects such as engineering bacteria to produce chimeric spider silk protein using hybrid genes assembled through techniques like the polymerase chain reaction (PCR). Early breakthroughs in genetic engineering involved the discovery and application of restriction endonucleases, such as EcoRI, which precisely cleave DNA at specific recognition sequences to produce fragments, sometimes leaving sticky ends for ligation. To clone eukaryotic genes lacking introns into microbial hosts, the enzyme reverse transcriptase is used to synthesize complementary DNA (cDNA) from processed mRNA templates. Successful DNA cloning requires a cloning vector (such as a plasmid, phage, cosmid, or artificial chromosome) that features an origin of replication, a selectable marker (like antibiotic resistance), and a multicloning site (MCS); these vectors are introduced into competent hosts like E. coli or Saccharomyces cerevisiae via transformation or electroporation. The Polymerase Chain Reaction (PCR) revolutionized gene manipulation by enabling the exponential amplification of targeted DNA sequences in vitro through repetitive cycles of denaturation, primer annealing, and DNA extension using thermostable polymerases; PCR is essential for diagnostics, forensics, and can be modified for seamless cloning techniques like Gibson assembly. For researchers seeking genes without prior sequence knowledge, genomic libraries are constructed by fragmenting an organism’s entire genome and cloning the pieces into vectors, which are then screened, often using phenotypic rescue or genetic complementation. To study or produce recombinant proteins, expression vectors are utilized, which contain strong, inducible promoters to control heterologous gene expression; these recombinant proteins can be easily purified by incorporating a polyhistidine (His)-tag, which selectively binds to metal resins. Additionally, protein function and location can be analyzed in vivo using fluorescent labeling, achieved by creating transcriptional fusions (to track promoter activity) or translational fusions (to determine subcellular localization). Finally, the chapter details the powerful CRISPR-Cas9 genome editing system, a precise tool where the Cas9 nuclease is directed by a specific guide RNA (gRNA) to create double-strand breaks in the target DNA. The cell repairs this damage either through error-prone nonhomologous end joining (NHEJ) or precisely via homologous recombination using a donor template, providing an exact method for altering genomes.