Transcription and RNA Processing

Figure 20.20 Summary of transcription, RNA processing and polypeptide synthesis. Polymerisation of the DNA template by RNA polymerase produces pre-mRNA (the primary transcript) this is transcription. The pre-mRNA is now processed, which involves capping, polyadenylation, editing and splicing (see text). The resultant mRNA transfers from the nucleus to the cytosol, where amino acids are polymerised to produce a polypeptide using the instructions present in the codons of the mRNA.

Gene expression can be controlled at multiple levels by changes in transcription, RNA processing, localization, and stabihty or utilization. Gene amphfica-tion and rearrangements also influence gene expression. 

In conjunction with studies performed by van Leeuwen et al. (135-138), Layfield et al. (263) proposed a novel mechanism that could account for an inhibition of 26S proteasome activity in cases of nonfamilial AD. Mutant forms of ubiquitin may inhibit proteolysis within neurons, predisposing these cells to inclusion formation. Molecular misreading of the UBB gene results in a dinucleotide deletion in UBB mRNA (135-138,264). In AD, an age-related posttranscriptional defect in primary transcript RNA processing may occur, leading to dinucleotide deletions within open reading frames that result in frameshifts and produce abnormal extension proteins, as demonstrated by van Leeuwen and coworkers (138). 

Kelley, D.E., Wiedemann, L.M., Pittet, A.-C., Strauss, S., Nelson, K.J., Davis, J., Van Ness, B., Perry, R.P. (1985). Nonproductive kappa immunoglobulin genes recombinational abnormalities and other lesions affecting transcription, RNA processing, turnover, and translation. Mol. Cell. Biol. 5,1660-1675. 

Eukaryotes have three varieties of RNA polymerase. These differ slightly from one another RNA polymerase II has die five core subunits plus the five common subunits, but also has two variable subunits specific to RNA polymerase II only. Note that RNA polymerase II has a tail, which is involved in RNA processing and the initiation of transcription (Fig. 11.7). 

Regulation may result from changes in genes or from mechanisms that operate at the level of transcription, during processing and transport of RNA, or at the level of translation. 

Capecitabine is a pyrimidine analog. It is an oral systemic prodrug that is enzymatically converted to 5-fluorouracil (5-FU). Healthy and tumor cells metabolize 5-FU to 5-fluoro-2-deoxyuridine monophosphate (FdUMP) and 5-flu-orouridine triphosphate (FUTP). These metabolites cause cell injury by two different mechanisms. First, they inhibit the formation of thymidine triphosphate, which is essential for the synthesis of DNA. Second, nuclear transcriptional enzymes can mistakenly incorporate FUTP during the synthesis of RNA. This metabolic error can interfere with RNA processing and protein synthesis. Capecitabine is indicated in the treatment of resistant metastatic breast cancer alone or in combination with docetaxel, and colorectal cancer. 

Tyrosine hydroxylase, the rate-hmiting enzyme, is a substrate for PKA, PKC, and CaM kinase phosphorylation may increase hydroxylase activity, an important acute mechanism whereby NE and Epi, acting at autoreceptors, enhance catecholamine synthesis in response to elevated nerve stimulation. In addition, there is a delayed increase in tyrosine hydroxylase gene expression after nerve stimulation, occurring at the levels of transcription, RNA processing, regulation of RNA stability, translation, and enzyme stability. Thus, multiple mechanisms maintain the content of catecholamines in response to increased transmitter release. In addition, tyrosine hydroxylase is subject to allosteric feedback inhibition by catecholamines. 

Recently, the related phenomenon of RNA interference (RNAi) has attracted much attention [5]. RNAi occurs when a short (generally 21 nucleotides in length) double-stranded RNA (dsRNA) catalyticaUy represses the translation of a fully complementary mRNA sequence. The process appears to proceed via a complex formed between the antisense RNA strand and a protein with RNase activity [6]. Upon binding to the target mRNA sequence, the ribonucleoprotein complex initiates cleavage of the mRNA transcript thus preventing translation of intact protein. After dissociation from the truncated mRNAs, the ribonucleoprotein complex is free to act on other intact mRNAs. Such small interfering RNAs (siRNAs) have... 

After transcription, during RNA processing, introns are removed and the exons are ligated together to form the mamre mRNA that appears in the cytoplasm. 

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