Bacterial Genetics and Functions Quiz

Microbial Genetics I

• Content Overview: The lecture covers microbial genetics, encompassing genetic elements (chromosomes and plasmids), DNA replication, transcription (in bacteria and archaea), translation, protein processing, secretion, targeting, and microbial regulatory systems.

Genetic Elements and DNA Replication

• Genetic Elements: Chromosomes are the primary genetic elements in bacteria and archaea. They are circular and typically contain most genes. Plasmids are smaller, circular DNA molecules that replicate independently of the chromosome. They often carry accessory genes. Transposable elements are sequences of DNA that can move within or between DNA molecules.

• DNA Replication: Understanding the mechanisms at the replication fork is important. DNA polymerase III is a key enzyme in replication. The leading strand is replicated continuously. The lagging strand is replicated discontinuously with Okazaki fragments. Circular DNA replicates in a theta structure.

Transcription

• Overview: Transcription is the process of synthesizing mRNA from DNA. Bacteria use RNA polymerase (core enzyme) and sigma factor for initiation. Archaeal RNA polymerase mechanism uses TBP (TATA binding protein) and TFB (Transcription factor B).

• Bacterial Transcription: Bacterial RNA polymerase, in combination with a sigma factor, recognizes promoter sequences. RNA polymerase moves along the DNA, temporarily opening the helix, and synthesizing RNA. Transcription continues until a termination sequence is reached. The termination mechanism can be rho-dependent or rho-independent.

• Archaea Transcription: Archaea RNA polymerase mechanism uses TBP (TATA binding protein) and TFB (Transcription factor B).

Translation

• Ribosomes: Bacterial and archaeal ribosomes have 30S and 50S subunits.

• Initiation Complex: The process starts with the binding of the initiator tRNA to the small ribosomal subunit. The large subunit joins to form the initiation complex.

• Elongation: mRNA codons are read, tRNAs bring corresponding amino acids, and peptide bonds are formed.

• Termination: Termination codons signal the end of translation.

Protein Processing, Secretion and Targeting

• Chaperones: These proteins facilitate proper protein folding and can help refold proteins that have misfolded. DnaK, DnaJ, and GroEL are examples.

• Sec System: The Sec system is commonly used to transport unfolded proteins across the cytoplasmic membrane or insert them into the membrane.

• Tat System: The Tat system transports folded proteins through the cytoplasmic membrane.

• Gram-negative Secretion System: Gram-negative bacteria utilize secretion systems (Types I-VI) to transport proteins or other small molecules, including effectors, to the outer membrane or outside the cell.

• Type III, IV, and VI Secretion Systems: These specialized systems commonly used by pathogenic bacteria inject proteins into eukaryotic host cells (Type III); transfer DNA between cells (Type IV); and inject toxins into neighboring cells/host (Type VI). T3SS ("injectisome") exhibits similarity in structure and function to a syringe.

Microbial Regulatory Systems

• Operons: Operons are clusters of genes under the control of a single promoter. Transcription occurs in a single mRNA molecule.

• Transcriptional Factors: These proteins control gene expression rates by binding to specific DNA sequences.

  • Transcriptional Regulation: Activation and repression of transcription are controlled by regulatory proteins like activators and repressors, that interact with DNA-binding sites (effectors). Activating proteins enhance transcription. Repressing proteins reduce transcription rates.

  • Effectors: Molecules like metabolites that bind to regulatory proteins and alter their ability to regulate gene expression. Inducers promote gene expression, and corepressors repress gene expression.

• Regulons: These are groups of operons that function as a single regulatory unit. (e.g., a series of operons involved in lactose or maltose metabolism).