NATIVE BOVINE SELENOCYSTEINE TRNA(SEC) SECONDARY STRUCTURE AS PROBED BY 2 PLANT SINGLE-STRAND-SPECIFIC NUCLEASES

Two single-strand-specific nucleases, discovered in plants, have been used to investigate the secondary and tertiary structures of the nativ e bovine liver selenocysteine tRNAS(Sec). To check the possible influe nce of nucleotide modifications on these structures, we compared the r esults obtained with the fully modified tRNA to the unmodified transcr ipt prepared by in vitro T7 transcription of the Xenopus laevis tRNA(S ec) gene. We found that the structures in solution of the native tRNA( Sec) and the transcript are Very similar despite some differences in a ccessibility to the enzymatic probes. Indeed, the modified anticodon-l oop of native bovine tRNA(Sec), containing 5-methylcarboxymethyluridin e (mcm(5)U(34)) and N-6-isopentenyladenosine (i(6)A(37)), is less acce ssible to Rn nuclease than that of the transcript: the intensity of ba nds representing cuts at A(36) and A(38) is much lower as compared to those of the transcript, whereas no cuts were found at the level of i( 6)A(37) in the anticodon loop of the native molecule, Surprisingly, th e variable arm of the native molecule has been found to be more suscep tible to single-strand-specific nuclease action, suggesting a looser s tructure of the variable arm in native bovine tRNA(Sec) than in the tr anscript.