Comparison of the acceptance activity of the ribosome-bound and the total cellular transfer ribonucleic acids from SV40-transformed mouse fibroblasts

Effects of liver regeneration on tRNA contents and aminoacyl-tRNA synthetase activities and sedimentation patterns

The tRNA content and aminoacyl-tRNA synthetases of regenerating liver in the phase of rapid growth were compared with those of livers from both intact and sham-operated rats. At 48 h after hepatectomy, the amount of active tRNA (called 'total acceptor capacity') is significantly higher in regenerating liver than in control livers, owing to a general, possibly not uniform, increase in the various tRNA families, which suggests that it may contribute to the increased protein synthesis and to decreased protein degradation as well. The activities of most, but not of all, aminoacyl-tRNA synthetases in cell sap of regenerating liver tend to be greater than normal. Increased activity of histidyl-tRNA synthetase fits in with the possibility that the mechanisms that control the rate of protein degradation through aminoacylation of tRNAHis in cultured cells [Scornik (1983) J. Biol. Chem. 258, 882-886] also operate in the liver and play a role in regeneration. Sedimentation analysis of cell sap in sucrose density gradients shows a shift of prolyl-tRNA synthetase activity toward the high-Mr form in regenerating liver. This change might be related to the positive protein balance and to growth in vivo, since it is also observed in the anaplastic Yoshida ascites hepatoma AH 130.

Nucleotide sequence of three isoaccepting lysine tRNAs from rabbit liver and SV40-transformed mouse fibroblasts

The lysine isoacceptor tRNAs differ in two aspects from the majority of the other mammalian tRNA species: they do not contain ribosylthymine (T) in loop IV, and a 'new' lysine tRNA, which is practically absent in non-dividing tissue, appears at elevated levels in proliferating cells. We have therefore purified the three major isoaccepting lysine tRNAs from rabbit liver and the 'new' lysine tRNA isolated from SV40-transformed mouse fibroblasts, and determined their nucleotide sequences. Our basic findings are as follows. a) The three major lysine tRNAs (species 1, 2 and 3) from rabbit liver contain 2'-O-methylribosylthymine (Tm) in place of T. tRNA1Lys and tRNA2Lys differ only by a single base pair in the middle of the anticodon stem; the anticodon sequence C-U-U is followed by N-threonyl-adenosine (t6A). TRNA3Lys has the anticodon S-U-U and contains two highly modified thionucleosides, S (shown to be 2-thio-5-carboxymethyl-uridine methyl ester) and a further modified derivative of t6 A (2-methyl-thio-N6-threonyl-adenosine) on the 3' side of the anticodon. tRNA3Lys differs in 14 and 16 positions, respectively, from the other two isoacceptors. b) Protein synthesis in vitro, using synthetic polynucleotides of defined sequence, showed that tRNA2Lys with anticodon C-U-U recognized A-A-G only, whereas tRNA3Lys, which contains thio-nucleotides in and next to the anticodon, decodes both lysine codons A-A-G and A-A-A, but with a preference for A-A-A. In a globin-mRNA-translating cell-free system from ascites cells, both lysine tRNAs donated lysine into globin. The rate and extent of lysine incorporation, however, was higher with tRNA2Lys than with tRNA3Lys, in agreement with the fact that alpha-globin and beta-globin mRNAs contain more A-A-G than A-A-A- codons for lysine. c) A comparison of the nucleotide sequences of lysine tRNA species 1, 2 and 3 from rabbit liver, with that of the 'new' tRNA4Lys from transformed and rapidly dividing cells showed that this tRNA is not the product of a new gene or group of genes, but is an undermodified tRNA derived exclusively from tRNA2Lys. Of the two dihydrouridines present in tRNA2Lys, one is found as U in tRNA4Lys; the purine next to the anticodon is as yet unidentified but is known not be t6 A. In addition we have found U, T and psi besides Tm as the first nucleoside in loop IV.

Role of methionine in the synthesis of nucleoside Q in Escherichia coli transfer ribonucleic acid

Previously, we reported that starvation of Rel Escherichia coli for methionine, but not leucine or histidine, results in chromatographically unique species of aspartyl-specific transfer ribonucleic acid (tRNAAsp) lacking the modified nucleoside Q. The present studies demonstrate that methionine starvation of Rel+ E. coli yields a qualitatively similar, but less pronounced, effect. Furthermore, during recovery from methionine starvation in Rel E. coli, the chromatographic elution pattern of tRNAAsp shifts towards that observed for unstarved cells after 1 h of recovery, and the shift appears complete after 2 h of recovery. This shift is inhibited by rifampin. Incorporation of [2-14C]methionine or [methyl-3H]methionine into growing cells of E. coli does not result in labeling of nucleoside Q. We interpret these findings to indicate that methionine has an indirect role in Q formation and that Q-deficient tRNA can be modified slowly to contain Q but that transcription is required. The chromatographic elution patterns of tRNAAsp from Rel E. coli starved for arginine, lysine, or glutamic acid indicate that these amino acids are not the source of the three- or five-carbon sequences in the modified portion of Q.

Inhibition of nucleoside Q formation in transfer ribonucleic acid during methionine starvation of relaxed-control Escherichia coli

The elution profiles of Asp-tRNA from unstarved and starved cultures of a relaxed-control (Rel-) strain of Escherichia coli were compared by reversed-phase chromatography. Methionine starvation results in the appearance of several additional species of Asp-tRNA which are not observed with starvation for leucine or histidine. By the criterion of cyanogen bromide-effected shifts in chromatographic elution position, a large portion of the tRNAAsp synthesized in methionine-starved cells lacks the normal Q nucleoside. By the same criterion, virtually all of the tRNAAsp from unstarved, leucine-starved, and histidine-starved cells contain Q. We conclude that methionine starvation prevents the formation of the norma Q nucleoside in Rel- E. coli.

Isoaccepting species differences between polysome-bound and total cellular tRNA in SVT2 cells

Reversed-phase chromatographic comparisons of total cellular tRNAs with tRNAs isolated from a polysome enriched cell fraction establish significant enrichments for specific isoaccepting species of tRNAIle, tRNALeu, tRNALys, tRNAMet and possibly tRNAA la. Similar comparisons of tRNAAsn, tRNAAsp, tRNAHis, tRNAPhe, tRNASer and tRNATyr have been performed as well; however, no prominent differences were observed.

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.

Alterations in SVT2 cell transfer RNAs in response to cell density and serum type

The chromatographic elution profiles of tRNA-A sn, tRNA-A sp, tRNA-H is and tRNA-T y r from SV40-transformed BALB-3T3 cells grown in fetal calf serum or cald serum-supplemented media have been examined. The relative proportions of certain of the isoaccepting species of these four tRNAs are altered in a similar fashion depending on the serum type. It is suggested that the elution profile alterations reflect the extent of modifications of a specific G residue to the minor nucleoside Q, and that this process differs between untransformed and transformed cells. In addition, cell density appears to influence the Q content of these tRNAs, though other density-dependent tRNA modifications also appear to occur.