A retroviral RNA secondary structure required for efficient initiation of reverse transcription

Genetic evidence is presented which suggests the existence of an important structural element in the 5' noncoding region of avian retrovirus RNA. The proposed structure, which we term the U5-leader stem, is composed of sequences in the middle of U5 and in the leader, flanking the primer-binding site. U5 and leader mutations which would disrupt this structure caused a partial replication defect. However, nucleotide substitutions in the leader, which would structurally compensate for a U5 deletion mutation, restored normal replication. Analysis of replication intermediates of viruses with the above mutations suggests that the U5-leader stem is required for efficient DNA synthesis in vivo and for initiation of DNA synthesis from the tRNA(Trp) primer in melittin-activated virions. However, this structure does not appear to be required for binding of the tRNA(Trp) primer to viral RNA. These results support a role for the U5-leader stem structure, independent of its primary sequence, in the initiation of retroviral replication.     

Rabbit liver tRNA1Val:II. unusual secondary structure of T psi C stem and loop due to a U54:A60 base pair.

In contrast to all other known tRNAs, mammalian tRNA1Val contains two adenosines A59 and A60, opposite to U54 and psi 55 in the U psi CG sequence of the T psi C loop, which could form unusual A:U (or A: psi pairs in addition to the five "normal" G:C pairs. In order to measure the number of G:C and A:U (A: psi) pairs in the T psi C stem, we prepared the 30 nucleotide long 3'-terminal fragment of this tRNA by "m7G-cleavage". From differentiated melting curves and temperature jump experiments it was concluded that the T psi C stem in this fragment is in fact extended by an additional A60:U54 pair. A dimer of this fragment with 14 base pairs was characterized by gel electrophoresis and by the same physical methods. An additional A:U pair in the tRNA1Val fragment does not necessarily mean that this is also true for intact tRNA. However, we showed that U54 is far less available for enzymatic methylation in mammalian tRNA1Val compared to tRNA from T-E. coli. This clear difference in U54 reactivity, together with the identification of an extra A60:U54 pair in the U psi CG containing fragment suggests the presence of a 6 base pair T psi C stem and a 5 nucleotide T psi C loop in this tRNA.     

Secondary structure in formylmethionine tRNA influences the site-directed cleavage of ribonuclease H using chimeric 2'-O-methyl oligodeoxyribonucleotides

In order to cleave RNA at specific positions in Escherichia coli formylmethionine tRNA, RNase H and complementary chimeric oligonucleotides consisting of DNA and 2'-O-methyl-RNA (Inoue et al. (1987) FEBS Lett. 215, 327] were used. Specific cleavages in the D loop, anticodon loop, T psi C loop, anticodon stem, and acceptor stem were investigated. Virtually unique hydrolyses with RNase H were observed at the T psi C loop, anticodon stem, and acceptor stem when relatively longer chimeric oligonucleotides (20-mer) were used. An efficient cleavage at the anticodon was obtained with a chimeric 13-mer when the higher structure of the tRNA was broken by hybridization with a 20-mer at the acceptor as well as the T psi C stem region. It was found that stabilities of hybrids with chimeric oligonucleotides and the presence of minor nucleosides affect the cleavage of tRNA by this approach.