RRNA methylation

Plasmid- or transposon-mediated resistance common to the MLS group is due to RNA methylase genes emiA, emiB and ermC) which code for the methylation of an adenine residue in 23 S rRNA. Methylation prevents the drugs from binding to the 508 ribosomal subunit and confers resistance to all MLS antibiotics. 

Finally, we propose that APR becomes phosphorylated by the putative APR-kinase April and thereby inactivated, as a second resistance mechanism in addition to the 16S rRNA methylation by KamB, during biosynthesis or thereafter. Because AprZ is significantly similar to the StrK protein, a member of the protein family of extracellular alkaline phosphates and a STR-phosphate-specific dephosphorylase (see Section 2.2.1.2), this modification is urgently suggested by presence of the conserved aprZ gene in the biosynthetic cluster. April is a member of the large kinase family comprising all the antibiotic and protein kinases. As in the STR producers, the postulated APR-phosphate would exported via the ABC transport system AprV/AprW and set free by dephosphorylation outside the cells via the phospatase AprZ. 

Maravic G. Macrolide resistance based on the Erm-mediated rRNA methylation. Curr. Drug Targets Infect. Disord. 2004 4(3) 193-202. 

In prokaryotic cells, 23S and 16S rRNA are formed in tandem the precursor molecule is only 10 % longer than the two mature rRNAs, and processing consists of the removal of 200 to 250 nucleotides from the 5 ends of the 23S and 16S precursors. Prokaryotic and eukaryotic 5S rRNA is always transcribed separately from the other rRNA. Methylated bases are characteristic components of all rRNA. 

Erythromycin Ribosomes from resistant cells have lower affinity, resulting from enzymatic methylation of adenine in 23S rRNA... 

Lawrence, R.J., Earley, K., Pontes, O., Sdva, M., Chen, Z.J., Neves, N., Viegas, W. and Pikaard, C.S. (2004) A concerted DNA methylation/histone methylation switch regulates rRNA gene dosage control and nucleolar dominance. Molecular Cell, 13, 599-609. 

This enzyme [EC 2.1.1.48] catalyzes the reaction of S-adenosyl-L-methionine with rRNA to produce 5-adeno-syl-L-homocysteine and rRNA containing A -methylade-nine. The enzyme will also methylate 2-aminoadenosine to 2-methylaminoadenosine. 

FIGURE 26-22 Processing of pre-rRNA transcripts in vertebrates. In step (T), the 45S precursor is methylated at more than 100 of its 14,000 nucleotides, mostly on the 2 -OH groups of ribose units retained in the final products. (5) A series of enzymatic cleavages produces the 18S, 5.8S, and 28S rRNAs. The cleavage reactions require RNAs found in the nucleolus, called small nucleolar RNAs (snoRNAs), within protein complexes reminiscent of spliceosomes. The 5S rRNA is produced separately. 

The sequences of all three pieces of RNA in the E. coli ribosomes are known as are those from many other species. These include eukaryotic mitochondrial, plas-tid, and cytosolic rRNA. From the sequences alone, it was clear that these long molecules could fold into a complex series of hairpin loops resembling those in tRNA. For example, the 16S rRNA of E. coli can fold as in Fig. 29-2A and eukaryotic 18S RNA in a similar way (Fig. 29-4).38/39/67 69 The actual secondary structures of 16S and 18S RNAs, within the folded molecules revealed by X-ray crystallography, are very similar to that shown in Fig. 29-2A. Ribosomal RNAs undergo many posttranscriptional alterations. Methylation of 2 -hydroxyls and of the nucleic acid bases as well as conversion to pseudouridines (pp. 1638-1641) predominate over 200 modifications, principally in functionally important locations that have been found in human rRNA.69a... 

E. coli has seven rRNA transcription units, each containing one copy each of the 23S, 16S and 5S rRNA genes as well as one to four tRNA genes. Transcription produces a 30S pre-rRNA transcript. This folds up to form stem-loop structures, ribosomal proteins bind, and a number of nucleotides become methylated. The modified pre-rRNA transcript is then cleaved at specific sites by RNase III and the ends are trimmed by ribonucleases M5, M6 and M23 to release the mature rRNAs. 

In summary, we have evidence from the short model sequences that modification according to the naturally occurring methylation pattern of helix 45 selects the observed secondary structure. To test the behavior of the fortymer terminal rRNA sequence of Bacillus stearothermophilus is the target of ongoing research. 

All classes of RNA transcripts must be processed into mature species. The reactions include several types Nucleolytic cleavage, as in the separation of the mature rRNA species from the primary transcript of RNA polymerase I action Chain extension (non-template-directed), as in the synthesis or regeneration of the common CCA sequence at the 3 end of transfer RNAs or of PolyA at the 3 end of mRNAs and Nucleotide modification, for example, the synthesis of methylated nucleotides in tRNA or rRNA. These reactions are a feature of both prokaryotic and eukaryotic gene expression, and the biological consequences are diverse. For example, modified nucleotides can affect the way in which a tRNA recognizes different codons. 

The replacement of thymine by uracil has no significant effect on the hydrogen bonding, as RNA does not use base pairing to form complementary dimers it is of less importance than it would be for DNA, but the removal of the methyl group may have an influence on the tertiary structures that RNA can adopt. From this it is clear that DNA is a better method of storing information whereas RNA is more suited to turn that information into a protein sequence. This is done by the ribosome, composed of ribosomal RNA (rRNA), which translates the codons of the mRNA sequence into a protein by matching three base sequences to those of tRNA that have the appropriate amino acids attached. 

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