Replication and pathogenicity of primer binding site mutants of SL3-3 murine leukemia viruses

Retroviral reverse transcription is primed by a cellular tRNA molecule annealed to an 18-bp primer binding site sequence. The sequence of the primer binding site coincides with that of a negatively acting cis element that mediates transcriptional silencing of murine leukemia virus (MLV) in undifferentiated embryonic cells. In this study we test whether SL3-3 MLV can replicate stably using tRNA primers other than the cognate tRNAPro and analyze the effect of altering the primer binding site sequence to match the 3' end of tRNA1Gln, tRNA3Lys, or tRNA1,2Arg in a mouse pathogenicity model. Contrary to findings from cell culture studies of primer binding site-modified human immunodeficiency virus type 1 and avian retroviruses, our findings were that SL3-3 MLV may stably and efficiently replicate with tRNA primers other than tRNAPro. Although lymphoma induction of the SL3-3 Lys3 mutant was significantly delayed relative to that of the wild-type virus, molecular tumor analysis indicated that all the primer binding site-modified viruses induce T-cell lymphomas similar to those induced by the wild-type virus in terms of frequencies of genomic rearrangements within the T-cell receptor beta-chain, the immunoglobulin kappa light chain, and the c-myc locus. Whereas none of the mutants were found to revert to tRNAPro primer utilization, in two tumors resulting from the injection of the SL3-3 Lys3 mutant the primer binding site was altered to match that of a new primer species, tRNA1,2Lys. In addition, recombination with endogenous viruses resulting in the generation of recombinant viruses carrying a glutamine primer binding site was detected in the majority of the tumors induced by the SL3-3 Lys3 mutant as well as in two tumors induced by wild-type SL3-3 and the SL3-3 Arg1,2 mutant.

Glutamyl-tRNA sythetase

Glutamyl-tRNA synthetase (GluRS) belongs to the class I aminoacyl-tRNA synthetases and shows several similarities with glutaminyl-tRNA synthetase concerning structure and catalytic properties. Phylogenetic studies suggested that both diverged from an ancestral glutamyl-tRNA synthetase responsible for the gluta-mylation of tRNA(Glu) and tRNA(Gln), and whose Glu-tRNA(Gln) product is transformed into Gln-tRNA(Gln) by a specific amidotransferase. This pathway is present in gram-positive and some gram-negative eubacteria, in some archae and in organelles, and was never found jointly with a glutaminyl-tRNA synthetase. Other gram-negative eubacteria and the cytoplasm of eukaryotes contain a glutamyl-tRNA synthetase specific for tRNA(Glu), and a glutaminyl-tRNA synthetase. Bacterial glutamyl-tRNA synthetases consist of about 500 amino acid residues, possess molecular masses of about 50 kDa, and act as monomers. In higher eukaryotes chimeric glutamyl-prolyl-tRNA synthetases were found, in a high molecular mass complex containing several other aminoacyl-tRNA synthetases. To date one crystal structure of a glutamyl-tRNA synthetase (Thermus thermophilus) has been solved. The molecule has the form of a bent cylinder and consists of four domains. The N-terminal half (domains 1 and 2) contains the 'Rossman fold' typical for class I synthetases and resembles the corresponding part of E. coli GlnRS, whereas the C-terminal half exhibits a GluRS-specific structure. As found for the other aminoacyl-tRNA synthetases the catalytic pathway of GluRS includes the formation of an aminoacyl adenylate in the first reaction step, but GluRS shares a special property with GlnRS and ArgRS: the ATP/PPi pyrophosphate exchange reaction is only catalyzed in the presence of the cognate tRNA. Compared with other aminoacyl-tRNA synthetases a relatively high number of investigations deals with recognition of tRNA(Glu) by GluRS. Besides interactions between the enzyme and the acceptor stem and the anticodon of tRNA(Glu), checking of the dihydrouridine arm and of the variable loop by GluRS are documented.

The transposable element Tn3 promotes general recombination at the neighboring regions

Transposon Tn3 was inserted into a tRNA operon of the amber suppressor Su+2 on a transducing phage (lambda hcI857nin5pSu+2) by selecting phages with ampicillin resistance and Su- phenotypes. In a strain thus obtained, Tn3 was inserted between the promoter and the first tRNA gene of the operon, which was determined by DNA sequencing. The Su+2 tRNA operon on the transducing phage consisted of two tRNA genes for tRNA(Met) and Su+2 tRNA(2Gln), which was a deletion derivative of the supB-E tRNA operon of E. coli containing seven tRNA genes in the order of promoter-Met-Leu-Gln1-Gln1-Met-Gln2-Gln2. Proliferating the lambda hcI857nin5pSu+2::Tn3 in E. coli cells, a number of phages which had lost Tn3 were isolated, and their tRNA gene compositions as well as the DNA structures of the tRNA operon were analyzed. In many cases the tRNA genes which had been deleted from the original transducing phage were regained from the chromosomal supB-E operon. Thus the loss of Tn3 from the phages was not due to excision of the transposon but due to the replacement of a portion of the tRNA operon, including Tn3, with the host homologous region that did not contain Tn3. This type of replacement takes place rather efficiently as a consequence of Tn3 insertion, owing to the general recombination occurring between homologous tRNA genes of phage and host chromosomes in the presence of either host recA or phage red. No such enhanced recombination in a similar cross between phage and host chromosomes was observed with the Tn3 present in the trans position on an independent plasmid. We conclude that inserting Tn3 in cis promotes general recombination in the neighboring regions. Possible mechanisms for this new type of genetic effect of Tn3 are discussed. During the course of this study, a natural defective mutation (T11) was also detected in one of the duplicated tRNA(2Gln) genes in an E. coli K12 strain we used.