RNA-Directed DNA Polymerases

DNA-directed RNA polymerase [EC 2.1.1.6] catalyzes the DNA-template-directed extension of the 3 -end of an RNA strand by one nucleotide at a time thus, n nucleoside triphosphate generate RNA and n pyrophosphate. The enzyme can initiate a chain de novo. Three forms of the enzyme have been distinguished in eukaryotes on the basis of sensitivity of a-amanitin and the type of RNA synthesized. See also Replicase... 

The rifamycins, a class of antibacterials isolated from Streptomyces mediterranei, contain a macrocyclic ring bridging across two nonadjacent positions on an aromatic system. Rifampicin (9.96), a semisynthetic derivative of rifamycin, is a drug of choice in the treatment of tuberculosis as well as leprosy, either alone or in combination with other drugs. Rifampicin is much safer than other antituberculotics since it inhibits DNA-directed RNA polymerase in bacteria but not in mammals. Another rifamycin, rifabutin (9.97), is a spiroimidazopiperidyl derivative of the rifamycin. 

Shortly after Ochoa s studies, Sam Weiss began a search for a DNA-directed RNA polymerase. His experimental design was influenced by the theory that RNA must be made on a DNA template if it is to carry the genetic message. He was also influenced by Komberg s discovery that DNA synthesis required nucleoside triphosphates for substrates rather than nucleoside diphosphates. With crude liver extracts, Weiss was able to demonstrate a capacity for RNA synthesis that was severely inhibited by DNase. Weiss s results touched off systematic investigations of RNA metabolism in many laboratories. A continuous expansion of research effort from that time on has yielded a wealth of understanding about the transcription process and related aspects of RNA metabolism. 

Weiss, Hurwitz, and Stevens discovered DNA-directed RNA polymerase. 

Nuclear genes encoding messenger RNAs for proteins are transcribed in eukaryotes by RNA polymerase II (Pol II). This is an enzyme complex comprising at least 12 subunits. Pol II, like the other DNA-directed RNA polymerases (I and III), cannot recognize... 

Most of the viruses that contain DNA (e.g., herpes) use this as a direct template for m-RNA production using host cell DNA-directed RNA-polymerase. However, the pox viruses have their own RNA polymerase. Finally, the so-called retroviruses like HIV use their RNA as a blueprint for the production of a complementary DNA strand with the help of an enzyme called reverse transcriptase - an RNA-dependent DNA polymerase. This nomenclature arises from the fact that these organisms operate in reverse of the normal cellular replicative process whereby DNA acts as a blueprint for RNA production. The DNA thus produced is incorporated into host cell DNA with the help of another viral enzyme - HIV integrase - and the cell ultimately produces new viral genomic RNA and viral proteins using its normal replication procedures. 

More specifically, it may be commented in passing that an enzyme involved in the formation of cancerous cells via viruses is DNA-dependent RNA polymerase, also called DNA-directed RNA polymerase (Hoffman, 1999, p. 189). On the other hand, what are called retroviruses have notably been implicated in cancer and immunodeficiency diseases. The enzyme involved here is called reverse transcriptase, or RNA-dependent DNA polymerase (or RNA-directed DNA polymerase). 

DNA-directed DNA polymerase DNA-directed RNA polymerase Dolichyl phosphatase Fatty acid enoyl reductase Viral neuraminidase Factor Xa... 

The antimicrobial activity of these antibiotics is due to their prevention of the growth of bacterial cultures by specifically inhibiting the activity of DNA-directed RNA polymerase (DDRP). Any modification in the ansa bridge generally reduces this activity. The interaction of rifamycin with bacterial RNA polymerase was discussed [214]. It was reported that electronegative groups at C-3 of rifamycin S enhance its activity toward DDRP [215]. 

A Concise Approach to (-)-Tirandamycin A (2) and (+)-Tirandamycic Acid (3). Tirandamycin A (2) is a representative member of a novel class of naturally-occurring antibiotics that are characterized by the presence of an enolized 3-dienoyl tetramic acid moiety coupled with the unusual dioxabicyclo[3.3.1]nonane ring system. In addition to its antimicrobial activity, 2 inhibits bacterial DNA-directed RNA polymerase, and it interferes with oxidative phosphorylation. Early synthetic work in this area focused upon the preparation of (+)-tirandamycic acid (3), " which is a degradation product of tirandamycin A (2). The first major achievement in this area was Ireland s synthesis of 3 from D-glucose, but more recently 2 has also been prepared by total synthesisOur own investigations in this area culminated in a facile, asymmetric synthesis of 26, which played a pivotal role in Ireland s synthesis of 2 and Schlessinger s synthesis of 3. ... 

The rifamycins act on the sensitive bacteria, blocking the synthesis of all cellular RNA, because they are potent inhibitors of all bacterial DNA-directed RNA polymerases. Most rifamycins are not effective on the mammalian RNA polymerase. Therefore, they possess the necessary requisite for ideal chemotherapeutic agents, that is, selective toxicity against pathogens. Detailed studies on the mechanism of action of rifamycins revealed that they inhibit the initiation of RNA synthesis by inactivating the enzyme before the incorporation of the first purine nucleotide of the RNA chain. Rifamycins form a rather stable equimolecular complex with the bacterial RNA polymerase, binding with the subunit of the enzyme [201-203]. 

During this period another enzyme captured our attention. Mike Chamberlin, as a graduate student, detected and purified an enzyme which synthesized RNA using DNA as a template the DNA-directed RNA polymerase of E. Thereupon followed a... 

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