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Category: Microbial Genetics and Molecular Biology
Glutamyl-tRNA as an Intermediate in Glutamate Conversions, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555818333/9781555810733_Chap27-1.gif /docserver/preview/fulltext/10.1128/9781555818333/9781555810733_Chap27-2.gifAbstract:
Since the discovery of initiator tRNA, a number of reactions have been found that involve modification of amino acids attached to tRNA. These reactions include well-understood processes like the formation of formylmethionyl-tRNA, which serves to initiate peptide chain formation in prokaryotes, or the synthesis of selenocysteine, which functions as the 21st amino acid. Less well understood are the conversions of glutamate. The first conversion serves for the formation of glutamate-1-semialdehyde, the initial precursor of porphyrins (e.g., chlorophylls and hemes). The other known reaction is a transamidation of glutamate attached to tRNAGln, which is a required intermediate in the formation of glutaminyl-tRNA in many organisms and organelles. These conversions of glutamate based on Glu-tRNA as intermediate are the topic of this chapter. Tetrapyrrole-containing compounds, such as hemes and chlorophylls, are essential components of respiratory and photosynthetic reactions. The porphyrin ring of these compounds is derived from eight molecules of 5-aminolevulinic acid (ALA), a precursor whose formation provides a key regulatory control point in heme and chlorophyll biosynthesis.
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The C5-pathway of ALA formation. Glu-tRNA reductase reduces Glu-tRNAGlu to glutamate 1-semialdehyde, with the release of free tRNAGlu. GSA is converted to ALA by glutamate 1- semialdehyde-2,l-aminomutase. The gene designations (for plant) and cofactor requirements are listed.
The C5-pathway of ALA formation. Glu-tRNA reductase reduces Glu-tRNAGlu to glutamate 1-semialdehyde, with the release of free tRNAGlu. GSA is converted to ALA by glutamate 1- semialdehyde-2,l-aminomutase. The gene designations (for plant) and cofactor requirements are listed.
Comparison of tRNAGlu sequences. The sequence of barley (Hordeum vulgare) chloroplast tRNAGlu is shown. Nucleotides in circled positions (variable positions) designate differences in primary structure among the chloroplast and Synechocystis sp. PCC 6803 tRNAGlu gene sequences compiled in Table 1 . Arrows indicate the differences (in addition to those found in the variable positions) between E. coli and barley chloroplast tRNAGlu.
Comparison of tRNAGlu sequences. The sequence of barley (Hordeum vulgare) chloroplast tRNAGlu is shown. Nucleotides in circled positions (variable positions) designate differences in primary structure among the chloroplast and Synechocystis sp. PCC 6803 tRNAGlu gene sequences compiled in Table 1 . Arrows indicate the differences (in addition to those found in the variable positions) between E. coli and barley chloroplast tRNAGlu.
The transamidation pathway of Gln-tRNA formation.
The transamidation pathway of Gln-tRNA formation.
Sequence alignment of chloroplast and Synechocystic 6803 tRNA genes a
a All sequences listed for comparison were obtained from GenBank or the EMBL data library. The organisms, with corresponding accession numbers, are as follows:Hordeum vulgare (XO6378),Oryza sativa (X15901),Triticum aestivum (X02805), Pinus radiate (L20419), Nicotiana tabaacum (X02217),Vicia faba (X00682), Pisum sativum (X01676), Spinacia oleracia (X04185), Epifagus virginiana (X61798), Pseudotsuga japonica (L20418), Pseudotsuga menziesii (L20417), Marchantia polymorpha (M20950), Euglena gracilis (J01412), Cyanophora paradoxa (M22563), Chlamydomonas reinhardtii (X54406), and Synechocystic sp. PCC 6803 (M32099). The secondary structure features of the tRNA are indicated.
Sequence alignment of chloroplast and Synechocystic 6803 tRNA genes a
a All sequences listed for comparison were obtained from GenBank or the EMBL data library. The organisms, with corresponding accession numbers, are as follows:Hordeum vulgare (XO6378),Oryza sativa (X15901),Triticum aestivum (X02805), Pinus radiate (L20419), Nicotiana tabaacum (X02217),Vicia faba (X00682), Pisum sativum (X01676), Spinacia oleracia (X04185), Epifagus virginiana (X61798), Pseudotsuga japonica (L20418), Pseudotsuga menziesii (L20417), Marchantia polymorpha (M20950), Euglena gracilis (J01412), Cyanophora paradoxa (M22563), Chlamydomonas reinhardtii (X54406), and Synechocystic sp. PCC 6803 (M32099). The secondary structure features of the tRNA are indicated.