Properties of isoaccepting species of lysine tRNA from rabbit reticulocytes in codon recognition and in haemoglobin biosynthesis in vitro

Perturbation of the mitochondrial lysine tRNA population by virus-induced transformation or stress of mammalian cells: functional properties and nucleotide sequence of a mitochondrially associated lysine tRNA

Eleven isoaccepting lysine tRNAs from mammalian sources are demonstrable by RPC-5 chromatography and polyacrylamide gel electrophoresis. The appearance and amounts of these isoacceptors varies with the source and growth state of cells. One isoacceptor, tRNALys6, observed in preparations of tRNA from some virus-transformed cells in culture, has been characterized by determining functional properties, cellular location, and its nucleotide sequence. tRNALys6 responds primarily to the lysine codon AAA, but it is not used efficiently in a wheat germ translational system in vitro. Compared with lysine isoacceptors 1, 2, 4, 5a, and 5, [3H]lysine appears in vivo in tRNALys6 with a delay of about 3 h. This delay may in part be a result of a less functional tRNA, but a compartmented state of tRNALys6 also appears to be important. tRNALys6 is associated with mitoplasts prepared from KA31 fibroblasts. The nucleotide sequence of tRNALys6 was determined by rapid postlabeling procedures involving limited hydrolysis in formamide, 32P-labeling of 5' ends of fragments with polynucleotide kinase, separation of the nested set of fragments in polyacrylamide denaturing gels, release of 5'-labeled nucleotides with RNase T2, and identification of the released nucleotides by chromatography on PEI cellulose. Confirmation of the positions of major nucleotides was done by using limited digestions by RNases of tRNALys6 labeled with 32P on the 3' terminus in a gel readout procedure. The nucleotide sequence of tRNALys6 differs from that of cytoplasmic lysine tRNAs and mammalian mitochondrial lysine tRNAs. It contains U*, an unidentified modified uridine occurring in the anticodon of some mitochondrial tRNAs. tRNALys6 appears to occur in very limited amounts, or not at all, in most cells unless stressed, but when present it is associated with mitochondria, although it is probably coded in the nucleus.

Codon binding and translational properties of an isoaccepting lysine tRNA peculiar to virus-transformed Cells

Isoaccepting lysyl-tRNAs from virus-transformed cells in culture were fractionated in the RPC-5 system into peaks 1, 2, 4, 5a, 5, and 6. tRNALys6 previously was found predominantly associated with transformed cells. The codon response of each peak was determined in an E. coli ribosomal binding assay. tRNALys1, tRNALys2, and tRNALys4 are highly specific for the 5'AAG3' codon. tRNALys5 and tRNALys5a preferentially bind in response to AAA. tRNALys6 binds in response to AAA 3-fold better than in response to AAG. The presence of thiolated nucleosides in the anticodon regions of tRNALys5a, tRNALys5, and tRNALys6 is indicated by I2-inactivation of aminoacylation ability with no effect on the other is isoacceptors. Functional abilities of the isoacceptors were compared in a wheat germ translational system with tobacco mosaic virus RNA as messenger. All of the isoacceptors function about equally well in translation except for tRNALys6, which is only 14 to 24% as effective as the other isoacceptors.

The methylation of tRNA

The methylation of tRNA is a post-transcriptional modification which is achieved by specific enzymes, the tRNA methylases, with S adenosylmethionine as a methyl donor. The level and pattern of methylation are characteristic of the tRNA species and origin. Abnormally methylated tRNAs have been obtained, in vivo and in vitro, by a variety of methods, and their properties have been studied. The tRNA methylases are found in all cells and tissues. Their activity varies with the differentiation state of the cells, and under the influence of many internal and external factors ; it is especially elevated in embryonic and cancerous tissues. These enzymes are very unstable, and none of them has been purified to homogeneity. We present here their known properties and we propose a theory concerning their specificity. Finally, after reviewing the few available experimental data, we discuss the current hypotheses and speculations about the roles and functions of tRNA methylation.

Relation of cell type and cell density to the degree of post-transcriptional modification of tRNALys and tRNAPhe

An examination of the reversed-phase chromatographic profiles of tRNALys and tRNAPhe from SV40-transformed BALB/3T3 cells grown to different cell densities, untransformed BALB/3T3 cells grown to confluency and BALB/c mouse liver indicates that with increasing cell density in culture the degree of the peroxy-Y modification in tRNAPhe and an undetermined modification in tRNALys become more like that of differentiated tissue (liver). Because precursor/product relationships appear to exist among the unmodified and modified forms of the isoaccepting species for each of these tRNAs, the present findings support the view that the often reported differences in tRNA isoaccepting spectra result primarily from differences in post-transcriptional modifications, rather than from different tRNA transcripts.

Nucleotide sequence analysis of RNA synthesized from rabbit globin complementary DNA

Rabbit globin complementary DNA made with RNA-dependent DNA polymerase (reverse transcriptase) was used as template for in vitro synthesis of (32)P-labeled RNA. The sequences of the nucleotides in most of the fragments resulting from combined ribonuclease T(1) and alkaline phosphatase digestion have been determined. Several fragments were long enough to fit uniquely with the alpha or beta globin amino-acid sequences. These data demonstrate that the cDNA was copied from globin mRNA and contained no detectable contaminants.

Coding properties of reticulocyte lysine transfer RNA's in hemoglobin synthesis

Two isoacceptor transfer RNA's for lysine were found in rabbit reticulocytes. The codon recognition properties of these isoacceptors were studied in hemoglobin synthesis in a cell-free systemn. The two isoacceptors transferred lysine into different sites in hemoglobin, but showed no preference for one chain over the other. Codon cross recognition was less than 4 percent.