Messenger RNA synthesis

The applications of FISH for chromosome-based studies involve the use of chromosome spreads, chromosomes of tissue sections, and interphase nuclei. Other applications include the study of adventitious organisms (2) and messenger RNA synthesis (3). The applications, especially to chromosome spreads, are sometimes enhanced by exploiting the opportunity presented by FISH for double or triple labeling. 

Dactinomycin is an antineoplastic agent that is the principal component of the mixture of actinomycins produced by Streptomyces parvullus. It inhibits messenger RNA synthesis. Dactinomycin (0.5 mg/day IV for 5 days), in combination with vincristine, radiotherapy, and surgery, is used in Wilms tumor in conjunction with vincristine, cyclophosphamide, and doxorubicin, it is used in choriocarcinoma in combination with methotrexate, it is used in testicular carcinoma and in combination with cyclophosphamide and radiotherapy, it is used in Ewing s sarcoma. Dactinomycin inhibits messenger RNA synthesis by anchoring to a purine-pyrimidine (DNA) base pair by intercalation (see Figure 15). The toxicity of dactinomycin increases when combined with radiation therapy. 

The antiviral mechanism of action of ribavirin relates to alteration of cellular nucleotide pools and inhibition of viral messenger RNA synthesis. Intracellular phosphorylation to the mono-, di-, and triphosphate derivatives is mediated by host cell enzymes. In both uninfected and RSV-infected cells, the predominant derivative (>80%) is the triphosphate, which has an intracellular t,/2 of elimination of less than 2 hours. 

Kates JR, McAuslan BR. Messenger RNA synthesis by a coated viral genome. Proc Natl Acad Sci USA. 1967 57 314-320. 

Italian scientists focussed their attention on the growth-promoting action of orotic acid in vitamin Bi2-deficient rats and chicks [24, 25]. Orotic acid and vitamin B12 have similar effects on the metabolism of the Cj-unit, whereby orotic acid increases the concentration of folate derivatives and influences the enzymes involved in the synthesis and utilization of folate intermediates [26,27]. Orotic acid also results in an increase in liver RNA concomitant with the stimulation of nuclear DNA-dependent RNA polymerase activity [28,29]. These findings led to the presumption that orotic acid increases messenger RNA synthesis. There are a number of further reports, mainly from Italy [3(M7], dealing with the relationship between orotic acid and vitamin deficiency, distribution and function [48 1]. 

Considerations of initiation of lac messenger RNA synthesis are directly related to the problems of induction (see Section IV). Since within 90 seconds of induction, enzyme appears, initiation of messenger synthesis must be rapid. The relative abundance of various messengers in the cell may be a function of the rate of initiation of RNA synthesis. For different RNA s the initiation rates vary by a wide margin. Thus initiation rates for ribosomal and tRNA synthesis in rapidly growing E. coli at 30°C range from 0.5 to 1 initiation per second [36], as contrasted with an initiation rate for the tryptophan operon mRNA of 1 every 2 to 5 minutes [37,38]. 

The studies outlined above indicate that a new model of catabolite repression involving a positive control system is beginning to emerge. This model can be summarized as follows In the presence of glucose or other catabolite repressor, the cellular level of cyclic AMP is reduced. This reduction leads to the inability of the cells to initiate lac messenger RNA synthesis. Cyclic AMP acts at the promotor locus, in conjunction with a protein, believed to be an activator protein for promoting initiation of RNA synthesis. 

DNA-dependent RNA synthesis is intriguing— especially messenger RNA synthesis in which it is almost certain that a single-stranded product is formed using a double-stranded primer. 

Interestingly, Rosenkranz et a/. have reported occasional false-positive results with Amitrole and have attributed them to phenotypic curing. It is not known whether this phenotypic curing is due to Amitrole acting as a histidine analogue in protein synthesis or as a ribonucleic acid analogue in messenger RNA synthesis. 

All the in vivo effects of actinomycin-D on bacteria or mammalian cells are accounted for by the affinity of the drug for DNA. and by its differential action on messenger-RNA synthesis, and on DNA metabolism itself. 

Control by Regulation of Messenger-RNA Synthesis the Model Proposed for Ecdysone... 

The chronology of the effects of ecdysone is also quite compatible with the model The hormone penetrates the epidermis cells within IS to 30 min after its injection, reaching a maximum concentration after 1 hr. Stimulation of messenger-RNA synthesis is seen within 1 to 2 hr. increase in labelling of microsomal RNA within 3 to 4 hr, and the enhancement of DOPA decarboxylase activity begins after 6 to 8 hr. 

The male sex hormone testosterone, as Williams-Ashman (1965) and Liao (1965) have shown, also acts on its target organs through stimulation of messenger RNA synthesis. It has been discovered no less clearly that the action of adrenal hormones, especially cortisone, is also associated with induction of certain changes in the genetic systems of cells. 

Figure 26-14 Simpiified picture of messenger RNA synthesis from a singie strand of (partly unwound) DNA.

AAV-specific RNA synthesis does not precede AAV DNA synthesis (Rose and Koczot, 1972). AAV RNA synthesis is first detectable approximately 2 hours after the onset of AAV DNA synthesis when AAV and adenovirus are used simultaneously to infect a cell. This is true even under conditions where adenovirus early messenger RNA synthesis is readily measurable. Thus the possibility exists that there is no class of AAV RNA synthesized prior to the onset of DNA replication. Again, if adenovirus infection preceded AAV infection by 10 hours, AAV-specific RNA could be detected starting at Shours after AAV infection at the time of onset of AAV DNA synthesis (Carter et al, 1973). 

By cell-free enzyme synthesis, the host shut-off was analysed and two mechanisms distinguished Shut-off of host translation and inhibition of host messenger RNA synthesis. 

Inhibition of host messenger RNA synthesis was determined using C-uridine pulses. The rate of RNA synthesis in the presence of nalidixic acid... 

Rouvi iRE, J., Wyngaarden, j., Cantoni, j., Gros, F., Kepes, a. Effect of T4 infection on messenger RNA synthesis in Escherichia coli. Biochim. biophys. Acta (Amst.) 166, 94-114 (1968). 

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