Termination of RNA synthesis

Termination of RNA synthesis occurs at specific sites along the DNA template. There are two modes of termination events those that require a termination protein, the p factor, and those that depend only on the transcription of regions of DNA containing stop signals. 

How does p provoke the termination of RNA synthesis A key clue is the finding that p hydrolyzes ATP in the presence of singlestranded RNA but not in the presence of DNA or duplex RNA. Hexameric p, which is structurally similar to and homologous to ATP synthase (Section 18.4.1), specifically binds single-stranded RNA a stretch of 72 nucleotides is... 

Termination of RNA synthesis occurs at specific base sequences within the DNA molecule. Many prokaryotic termination sequences have been determined and most have the following three characteristics (Figure 25-5) ... 

The enzyme uses one strand of the DNA as the template for RNA synthesis. The base sequence of the DNA contains signals for initiation and termination of RNA synthesis. The enzyme binds to the template strand and moves along it in the 3 -to-5 direction. 

The synthesis of mRNA terminates in the polychromatophilic erythroblast. Thus, no RNA is synthesized in reticulocytes and orthochromic erythroblasts. The mRNA synthesized in the basophilic erythroblast is stable and remains in the cell for long periods. This mRNA is active even in the reticulocyte stage. Protein synthesis starts after the termination of RNA synthesis in the polychromatophilic erythroblast, The rate of hemoglobin synthesis is maximal in polychromatophilic and orthochromic erythroblasts (Marx and Kovach, 1966). There is a correlation between the number of polysomes and the rate of hemoglobin synthesis (Table 40). This was demonstrated with the aid of electron microscopy and direct visualization of polysomes and separate ribosomes. 

Other steps of RNA synthesis such as elongation or termination also control the abundance of proteins. These are less well understood than the initiation of transcription described here. 

Our discussion of RNA synthesis begins with a comparison between transcription and DNA replication (Chapter 25). Transcription resembles replication in its fundamental chemical mechanism, its polarity (direction of synthesis), and its use of a template. And like replication, transcription has initiation, elongation, and termination phases—though in the literature on transcription, initiation is further divided into discrete phases of DNA binding and initiation of RNA synthesis. Transcription differs from replication in that it does not require a primer and, generally, involves only limited segments of a DNA molecule. Additionally, within transcribed segments only one DNA strand serves as a template. 

The process of RNA synthesis is called transcription. The enzyme that synthesizes RNA is RNA polymerase, which is a multisubunit enzyme. The core enzyme has four subunits—2 a, 1 p, and 1 p, and possesses 5 —>3 polymerase activity. The enzyme requires an additional subunit—sigma (a) factor—that recognizes the nucleotide sequence (promoter region) at the beginning of a length of DNA that is to be transcribed. Another protein—rho (p) factor—is required for termination of transcription of some genes. 

Initiation factors contribute to the ribosome complex with the messenger RNA and the initiator methionyl-tRNA. Elongation factors assist the binding of all the other tRNAs and the translocation reaction that must occur after each peptide bond is made. Termination factors recognize a stop signal and lead to the termination of polypeptide synthesis and the release of the polypeptide chain and the messenger from the ribosome. 

The three phases of RNA synthesis are initiation, elongation, and termination of... 

TFIIE, TFIIF, TFIIH, TFIIJ, and TFIIA. Phosphorylation of the carboxy terminal repeat domain (CTD) of the RNA pol II large subunit is associated with initiation of RNA synthesis. 

The chapter begins -with an overview of the three stages of RNA synthesis initiation, elongation, and termination. The subunit structure of RNA polymerase from... 

Animals which have not ovulated recently contain a population of oocytes, usually larger in diameter than 1.2 mm, which exhibit unpig-mented equatorial bands. These stage 6 oocytes (Dumont, 1972) are considered to represent the terminal stage of oocyte development and are always responsive to hormonal stimulation, both in vivo and in vitro. While these are not the only oocytes capable of undergoing maturation, the white band provides a convenient marker for isolation, and such oocytes have been utilized in our studies of RNA synthesis. For comparison, oocytes between 0.5 and 0.6 mm diameter (lampbrush chromosome stage 4 according to Davidson and Mirsky, 1965) were also studied. 

RtilMse factors, urmimtum factors catalytically active proteins necessary for the termination step of RNA synthesis and protein biosynthesis. For further details, see Ribonucleic add, otein biosynthesis (subheading Termination). 

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