Posttranscription modification

Most often proteins are the bacterial biopolymers studied using MALDI MS either from fractions or whole cells. They are not the only isolated cellular biopolymers studied by MALDI, nor the first. Very soon after the introduction of MALDI there were a few reports of the analysis of bacterial RNA or DNA from bacterial fractions. One of the first applications of MALDI to bacteria fractions involved analysis of RNA isolated from E. coli,4 Other studies included analysis of PCR-amplified DNA,5 6 DNA related to repair mechanisms7 and posttranscriptional modification of bacterial RNA.8 While most MALDI studies involve the use of UV lasers, IR MALDI has been reported for the analysis of double stranded DNA from restriction enzyme digested DNA plasmids, also isolated from E. coli.9... 

Kirpekar, F. Douthwaite, S. Roepstorff, P. Mapping posttranscriptional modifications in 5S ribosomal RNA by MALDI mass spectrometry. RNA 2000, 6,296-306. 

Introns in DNA can be visualized in an electron micrograph of DNA-mRNA hybrids (Figure 1-3-8). When mRNA hybridizes (base pairs) to the template strand of DNA, the introns appear as unhybridized loops in the DNA. The poly-A tail on the mRNA is also unhybridized, because it results from a posttranscriptional modification and is not encoded in the DNA. 

These processes are summarized in Figure 28-1. We have examined several of these mechanisms in previous chapters. Posttranscriptional modification of mRNA, by processes such as alternative splicing patterns (see Fig. 26-19b) or RNA editing (see Box 27-1), can affect which proteins are produced from an mRNA transcript and in what amounts. A variety of nucleotide sequences in an mRNA can affect the rate of its degradation (p. 1020). Many factors affect the rate at which an mRNA is translated into a protein, as well as the posttranslational modification, targeting, and eventual degradation of that protein (Chapter 27). 

A. Primary tRNA transcript. B. Functional tRNA after posttranscriptional modification. Modified bases include D (dihydrouridine), (pseudouridine), and m. which means that the base has been methylated. 

An intron (see below) must be removed from the anticodon loop, and sequences at both the 5 - and the 3 -ends of the molecule must be trimmed. Other posttranscriptional modifications include addition of a -CCA sequence by nucleotidyltransferase)to the 3 -terminal end of tRNAs, and modification oTbases at specific positions to produce "unusual bases (see p. 290). 

Posttranscriptional modification of mRNA showing the 7-methylguanosine cap and poly-A tail. 

All tRNAs contain several ribonucleotides that differ from the usual four (12 in the case of tRNAphe). The structures for some of these are shown in figure 28.4. Only four ribonucleotides are incorporated into RNA in the transcription process. All of the rare bases found in the mature tRNA result from posttranscriptional modification. 

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