Elongation of bacterial DNA

At least six different proteins are involved in the elongation of bacterial DNA (Table 8-2). 

So this research reveals new mechanisms of bacterial autoregulation under extreme conditions, controlled by low weight molecules - alkyiresorcinols. It applied aspects are defined by developing of methods for DNA protection in vitro and elongators of bacterial cells viability at UV exposure. 

Compare systhesis of the leading and lagging strands in the elongation phase of DNA replication. Explain why DNA polymerases may have difficulty in replicating the 3 -end of the lagging strand of linear DNA. How has this problem been solved in many bacterial and viral systems In eukaryotic cells ... 

Rifampicin is a synthetic derivative of a naturally occurring antibiotic, rifamycin, that inhibits bacterial DNA-dependent RNA polymerase but not T7 RNA polymerase or eukaryotic RNA polymerases. It binds tightly to the ft subunit. Although it does not prevent promoter binding or formation of the first phosphodiester bond, it effectively prevents synthesis of longer RNA chains. It does not inhibit elongation when added after initiation has occurred. Another antibiotic, streptolydigin, also binds to the ft subunit it inhibits all bond formation. 

Enzymatic replication of DNA is a complex process and requires the cooperation of some 20 or more proteins. Arthur Komberg and his colleagues first discovered an enzyme in E. coli that catalyzed the polymerization of deoxyribonucleotides under the direction of a DNA template. This enzyme, DNA polymerase I (Pol I), is now known to be only one of a group of similar enzymes that can copy DNA or RNA templates or both. Three distinct enzymes (Pol I, Pol II, and Pol III) have been isolated from bacterial cells (Table 11.1). These enzymes catalyze the stepwise addition of deoxyribonucleotide residues to the free 3 -hydroxyl end of a preexisting DNA or RNA primer strand thus the enzymatic elongation proceeds in the 5 —> 3 direction. The overall reaction is... 

Furthermore, in the case of rifampicin (=rifampin, a synthetic rifamycin derivative) the resistance was localized in the p-subvinit of the RNA-poly-merase. Binding of rifamycin to RNA-polymerase from bacterial sensitive strains has been found under varied experimental conditions, while the antibiotic does not bind to the enzyme from resistant strains, Rifamycin does not prevent the interaction of RNA-polymerase with template DNA , The antibiotic blocks the addition of the first ribonucleotide at the initiation of synthesis of an RNA chain by RNA-polymerase. but it cannot inhibit the synthesis of an RNA chain once it has been started On the other hemd, streptolydigin appears to inhibit elongation of RNA syn-thesis , RNA-polymerase from rat liver mitochondria has also been shown to be sensitive to rifamycin . The RNA-polymerase from blue-green algae was found to be sensitive to rifamycin, but to a lesser extent than the bacterial enzyme... 

Rifampicin and other members of the rifamycin group bind to the subunit of bacterial RNA polymerase and block the formation of the first phosphodiester bond. They therefore inhibit initiation, with little effect on elongation or termination of previously initiated chains. Actinomycin D binds tightly to double-stranded DNA and prevents it from acting as a template for transcription. It binds between adjacent base pairs, particularly in G-rich regions, by a process known as intercalation. At low concentration, RNA synthesis is effectively inhibited with little effect on DNA or protein synthesis. 

The drug rifampin binds to bacterial RNA polymerases and is a useful experimental inhibitor of initiation of transcription. It binds to the P subunit of RNA polymerase, blocking the transition from the chain initiation phase to the elongation phase it is an inhibitor of chain initiation but not of elongation. Actinomycin D also inhibits initiation but does so by binding to DNA. These drugs have limited clinical use because of their toxicity. 

MECHANISM OF ACTION AND BACTERIAL RESISTANCE Rifampin forms a stable complex with DNA dependent RNA polymerase, leading to suppression of initiation of chain formation (but not chain elongation) in RNA synthesis. High concentrations of rifampin can inhibit RNA synthesis in mammahan mitochondria, viral DNA-dependent RNA polymerases, and reverse transcriptases. Rifampin is bactericidal for both intracellular and extracellular microorganisms. 

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