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The nucleotide sequence of a methionine tRNA which functions in protein elongation in mouse myeloma cells
Abstract:
The major form of methionine tRNA operational in the elongation of protein synthesis in mouse myeloma cells was purufied from these cells after they had been cultured in the presence of [32P]-phosphate. This [32P]tRNA4-Met species was then digested with T1 RNase or pancreatic RNase so as to obtain both complete and partial RNase digestion products. The nucleotide sequences of these fragments were analysed to enable the derivation of the complete primary structure of this tRNA. tRNA4-Met of mouse myeloma cells is 76 nucleotides in length and contains 15 modified nucleotides. It is the only tRNA yet sequenced which has been found to possess the minor nucleoside 2-methylguanosine (m2G) within the amino acid (a) stem, and also to have an anticodon (c) stem of only 4 and not 5 base-pairs. The loop IV sequence of eukaryotic initiator methionine tRNA (tRNAf-Met) species, -A-U-C-G-m1A-A-A-, IS NOT FOUND IN TRNA4-Met and is therefore absent from at least one of the methionine tRNAs functioning in polypeptide elongation in mammalian cells. This is consistent with the suggested importance of this loop structure in the initiator function of tRNAf-Met in eukaryotic organisms. Three distinct regions of the tRNA cloverleaf, the (b) stem, the anticodon loop (loop II), and loop III, are substantially conserved in structure between tRNAf-Met and tRNA4-Met of mouse myeloma cells. These regions of the structures of mammalian methionine tRNAs probably do not determine whether a certain tRNA-Met will function in the initiation or elongation of protein synthesis, although they might be important in tRNA-Met recognition if the different cytoplasmic tRNA-Met species of mammalian cells are aminoacylated by a single activating enzyme.
Insights
Researchers purified methionine transfer RNA (tRNA4-Met) from mouse myeloma cells to determine its primary structure. This analysis revealed unique features, including a modified nucleoside and a shorter anticodon stem, differentiating it from initiator tRNA.
Area of Science:
- Molecular Biology
- Biochemistry
- Genetics
Background:
- Methionine transfer RNA (tRNA-Met) plays crucial roles in both protein synthesis initiation and elongation.
- Understanding the structural diversity of tRNA-Met is essential for elucidating their specific functions in cellular processes.
Purpose of the Study:
- To determine the complete primary nucleotide sequence of the major methionine tRNA (tRNA4-Met) involved in protein synthesis elongation in mouse myeloma cells.
- To compare the structure of tRNA4-Met with that of eukaryotic initiator methionine tRNA (tRNAf-Met) to identify potential functional distinctions.
Main Methods:
- Purification of tRNA4-Met from mouse myeloma cells cultured with [32P]-phosphate.
- RNase T1 and pancreatic RNase digestion of purified tRNA4-Met to generate complete and partial digestion products.
- Analysis of nucleotide sequences of RNase digestion fragments to derive the primary structure.
Main Results:
- The primary structure of mouse myeloma tRNA4-Met was determined; it is 76 nucleotides long and contains 15 modified nucleotides.
- tRNA4-Met possesses the minor nucleoside 2-methylguanosine (m2G) in the amino acid stem and a 4 base-pair anticodon stem, unlike other sequenced tRNAs.
- The characteristic loop IV sequence of eukaryotic tRNAf-Met is absent in tRNA4-Met, supporting its role in elongation rather than initiation.
Conclusions:
- Mouse myeloma tRNA4-Met has a unique primary structure with distinct features compared to tRNAf-Met.
- The absence of the loop IV sequence in tRNA4-Met supports its role in polypeptide elongation.
- Conserved regions in the (b) stem, anticodon loop, and loop III suggest these may not solely determine initiation versus elongation function but could be involved in tRNA-Met recognition by aminoacyl-tRNA synthetases.