Introduction to Microbiology Anas Abu-Humaidan M.D. Ph.D. Lecture 7

Top Adenine thymidine (A-T) pairing; bottom: guanine-cytosine (G-C) pair 100s-1000s kbp Uracil in ribonucleic acid, or Thymidine in deoxyribonucleic acid G+C contect

Bacterial DNA Circular chromosomal and extra chromosomal (plasmids, bacteriophages) replicons. Supercoiled DNA…. No histones, polyamins Haploid smaller… mutations The majority of antibiotics that target the 30S subunit inhibit protein synthesis by preventing either the binding of tRNAs to the ribosome or the movement of tRNAs through the ribosome during translocation …. Aminoglycosides and tetracyclines

Transcription and translation the DNA double helix if read in the3′–5′ direction. Thus, an mRNA is oriented in a 5′–3′direction. No nulear membrane couple transcription, translation No poly adenalation (tRNA and rRNA genes mostly at any point) , no introns Cistrons /genes mostly functional unlike eukaryotes (coding structural proteins genes … mRNA) Rho-independent termination is controlled by specific sequences in the DNA template strand. As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C–G nucleotides. The mRNA folds back on itself, and the complementary C–G nucleotides bind together. The result is a stable hairpin

Transcription and translation RNA polymerase The enzyme peptidyltransferase (which is actually the 23S rRNA, ie, a ribozyme) catalyzes the formation of the peptide bond

Regulation of transcription prokaryotic structural genes that encode a related series of metabolic reactions are clustered on operons …… polycistronic mrna

Regulation of transcription Repressors are proteins that suppress transcription of a gene in response to an external stimulus, whereas activators are proteins that increase the transcription of a gene in response to an external stimulus. Finally, inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate

Regulation of transcription Repressors are proteins that suppress transcription of a gene in response to an external stimulus, whereas activators are proteins that increase the transcription of a gene in response to an external stimulus. Finally, inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate

Regulation of transcription

DNA replication

DNA replication

DNA replication Because bacterial topoisomerases are essential and unique, they are targets of antibiotics (eg, quinolones).

DNA mutations missense mutation results in a different amino acid being inserted in the protein, but this may be a conservative mutation if the new amino acid has similar properties ….. Null mutations redenreds a gene non/functional Exogenous triggers include UV irradiation, chemicals or oxidative compounds, acids, organic mutagens, some antibiotics 

DNA mutations repair 1. Direct DNA repair is the enzymatic removal of damage, such as pyrimidine dimers and alkylated bases. 2. Excision repair is the removal of a DNA segment containing the damage, followed by synthesis of a new DNA strand. 3. Recombinational or postreplication repair replaces a missing or damaged section of DNA with the same or similar sequences that may be present during replication or on extrachromosomal DNA. 4. The SOS response is the induction of many genes (≈15) after DNA damage or interruption of DNA replication to promote recombination or error-prone repair. 5. Error-prone repair is the last resort of a bacterial cell before it dies. It is used to fill in gaps with a random sequence when a DNA template is not available for directing an accurate repair.

Further reading: Murray - Medical Microbiology 8th Edition Section 4: Bacteriology Chapter 13: Bacterial metabolisim and genetics Jawetz, Melnick & Adelberg's Medical Microbiology, 26th edition- Section 1: Fundamentals of Microbiology- Chapter 7:Microbial genetics