Poly polymerase enzyme-linked

Mature mRNA transcripts (sense strand) from eukaryotic cells can be purified and then reverse transcribed, with the assistance of a reverse transcriptase enzyme (from Moloney murine leukemia virus, MMLV), into complementary DNAs (cDNAs) that will anneal with the mRNA transcripts by Watson-Crick base pairing to give anti-parallel DNA/RNA duplexes or double helices. The poly(A) tail in each mature mRNA transcript is actually a usefiil handle for each reverse transcriptase reaction. Thereafter, DNA/RNA duplexes must be broken down with the assistance ofRNAse enzymes (specific for the hydrolysis of RNA phospho diester links) and a sense strand of DNA constructed instead on each cDNA single strand so that equivalent, more stable antiparallel DNA/DNA duplexes are generated instead, with the assistance of a DNA polymerase enzyme. In this instance, the poly(T) tail in each cDNA molecule turns out to be important for the DNA polymerase reaction ... 

In the synthesis route from acetyl-CoA to poly(3HB), at least three steps and three enzymes are involved (Fig. 1). The first step is catalyzed by the 3-keto-thiolase (EC 2.3.1.9) which reversibly links two acetyl-CoA moieties to aceto-acetyl-CoA in a Claisen-condensation. The conversion of acetoacetyl-CoA into D-(-)-3-hydroxybutyryl-CoA can be mediated by a reductase (step 2) or via a sequence catalyzed by a reductase (step 4) and two hydratases (steps 5,6). The last step, i.e., the polymerization, is catalyzed by a polymerase (step 3). This... 

Poly(ADP-ribosyl)ation is a posttranslational protein modification carried out by a family of enzymes, referred to as poly(ADP-ribose) polymerases (PARPs). The biochemistry of poly(ADP-ribose) formation and degradation, and the molecular and cell biolc of the enzymes involved are described in detail in several other Chapters. However, it is worth summarizing several facts that are particularly relevant for the link between pol)r(ADP-ribosyl)ation and aging. 

The poly(ADPR) polymerase was stimulated by histone HI when the HI enzyme ratio was of 2 histones H2A, H2B, H3, and H4 had little effect on the sDNA-linked enzyme [16-18]. The specific sDNA fraction also protects some enzyme sites against inhibition. Ultraviolet spectroscopic properties of sDNA suggested the presence of single stranded or denaturated DNA portions in sDNA. The Km for NAD was shown to vary with the DNA concentration and two apparent Km could be defined, depending on the DNA concentration. It may correspond to some conformational changes in the enzyme or in the enzyme-DNA complex. The ratio of the apparent K for sDNA to the enzyme concentration is constant at any enzyme level. Moreover, the minimum estimation of the number of base pairs of sDNA required for maximal activation of one enzyme molecule was 16, while for calf thymus DNA this estimation was 640 [16]. 

In the late 1970 s as evidence started to mount [1,2] linking poly(ADP-ribose) polymerase to DNA repair, this laboratory started to explore the possible clinical significance of this nuclear enzyme. A number of papers had already been published concerning the effects of carcinogens on the metabolism of nicotinamide, NAD, and poly(ADP-ribose). These results can be summarized as follows ... 

As synthesis of RNA proceeds in the 5 to 3 direction, transcription of the viral RNA must initiate at the 3 end of the template, that is, on the poly(A) tract. A poly(B) polymerase hence appears quite likely to be the enzyme responsible for initiating the synthesis of the minus strand. The need for an oligo(u) primer, however, makes that enzyme dependent on another one (be it of cellular or viral origin) which in t irn must provide the primer. Alternatively, the requirement of a pre-formed oligo(u) primer might simply reflect the inability of the m vitro system to initiate transcription in the absence of a UTP residue linked to the small viral protein VPg (chapter 9) ... 

DNA synthesis takes place by a mechanism of complementary autoreplication. DNA is present in nearly all biological systems (with the exception of certain phages) as a double helix, formed by poly-desoxyribonucleotides, linked together by complementary hydrogen bonds between base pairs (A-T) (G-C), twisted aroxmd a common axis. Under the influence of a special enzyme, polymerase, it can unwind into single polynucleotides, each of which becomes a template for formation of the complementary chain. This process is shown diagrammatically in Fig. 1. 

---