Alkylation of tRNATrp in a complex with tryptophanyl-tRNA synthetase

Interaction of tRNAPhe and tRNAVal with aminoacyl-tRNA synthetases. A chemical modification study

The alkylation by ethylnitrosourea of phosphodiester bonds in tRNAPhe from yeast and in tRNAVal from yeast and from rabbit liver and that by 4-(N-2-chloroethyl-N-methylamino)-benzylamine of N-7 atoms of guanosine residues in yeast tRNAVal have been used to study the interaction of these tRNAs with aminoacyl-tRNA synthetases. The modifications occurring at low yield were carried out on 3' and/or 5' end-labelled tRNAs either free or in the presence of cognate or non-cognate synthetases. After splitting of the tRNAs at the alkylated positions, the position of the modification sites in the tRNA sequences were detected by acrylamide gel electrophoresis. It was found that the synthetases protect against alkylation certain phosphate or guanosine residues in their cognate tRNAs. Non-cognate synthetases failed to protect efficiently specific positions in tRNA against modification. In yeast tRNAPhe the cognate phenylalanyl-tRNA synthetase protects certain phosphates located in all four stems and in the anticodon and extra-loop of the tRNA. Particularly strong protections occur on phosphate 34 in the anticodon loop and on phosphates 23, 27, 28, 41 and 46 in the D and anticodon stems. In yeast tRNAVal complexed with yeast valyl-tRNA synthetase the protected phosphates are essentially located in the corner between the amino-acid-accepting and D stems, in the D loop, anticodon stem and in the variable region of the tRNA. Three guanosine residues, located in the D stem, and another one in the 3' part of the anticodon stem were also found protected by the synthetase. In mammalian tRNAVal, complexed with the cognate but heterologous yeast valyl-tRNA synthetase, the protected phosphates lie in the anticodon stem, in the extra-loop and in the T psi arm. The location of the protected residues in the structure of three tRNAs suggests some common features in the binding of tRNAs to aminoacyl-tRNA synthetases. These results will be discussed in the light of informations on interaction sites obtained by nuclease digestion and ultraviolet cross-linking methods.

A rapid method for mapping exposed cytosines in polyribonucleotides. Application to tRNATrp (yeast, beef liver)

A rapid method for mapping exposed cytosine residues in 5'-[32P]-labeled RNA molecules is suggested. The exposed cytosines (C's) are converted into uracyls (U's) by bisulphite treatment at pH 5.8 in the presence of Mg2+, followed by complete modification of the residual (non-exposed) C's by a methoxyamine and bisulphite mixture at pH 5.0. The control RNA is modified only by methoxyamine and bisulphite without the preliminary C leads to U conversion. The location of the exposed C's is determined by comparing the products of partial T1, T2, A and U2 ribonuclease digestions of the C leads to U converted and control RNAs after slab gel polyacrylamide electrophoresis and autoradiography. The method has been applied for mapping exposed cytosine bases in tRNATrp (yeast) which have been found in the anti-codon loop and at the 3'-end of the molecule. In tRNATrp (beef liver), in addition to the same exposed bases, C in the diHU-loop is exposed. The data obtained are in full agreement with what is known about exposed C's for other tRNAs.

Immunochemical studies of beef pancreas tryptophanyl-tRNA synthetase and its fragments. Determination of the number of antigenic determinants and a comparison with tryptophanyl- tRNA synthetases from other sources and with reverse transcriptase from avian myeloblastosis virus

The immunoglobulin G (IgG) fraction of the antiserum from rabbits immunized with homogeneous beef pancreas tryptophanyl-tRNA synthetase inhibits the enzyme activity in the reactions of both tRNATrp aminoacylation and tryptophan activation. Fab fragments of IgG act in a similar way. Common antigenic determinants have been detected in tryptophanyl-tRNA synthetases from beef, pig, chicken and rat livers using pure antibodies against beef pancreas tryptophanyl-tRNA synthetase. This observation indicates the evolutional stability of certain structural features of tryptophanyl-tRNA synthetases. The interaction of antibodies with the fragments of beef tryptophanyl-tRNA synthetase produced by endogenous and tryptic proteolysis of the enzyme has been studied. On third of the antiserum antibodies interacting with the C-terminal fragment of the enzyme (Mr approximately equal to 40000) inhibits its activity whereas the antibodies to the N-terminal fragment (Mr approximately equal to 20000) have no effect on the enzyme activity. The immunochemical identity of the two synthetase fragments differing in their enzymatic activity supports the assumption that the loss of enzymatic activity of the tryptic fragment is caused by lack of a small peptide which is retained in case of endogenous proteolysis; probably the amino acid residues of this peptide participate in formation of active centre of tryptophanyl-tRNA synthetase. A radioimmunochemical method is described for determining the number of antigenic determinants. One molecule of tryptophanyl-tRNA synthetase was found to bind 9 (+/- 1) molecules of Fab fragments. Antibodies against tryptophanyl-tRNA snythetase from beef pancreas do not inhibit noticeably the activity of reverse transcriptase from avian myeloblastosis virus. No antigenic determinants in common have been detected in reverse transcriptase and tryptophanyl-tRNA synthetase by radioimmunochemical assays.