The viroid of the potato spindle tuber disease (PSTV) is a covalently closed ring of 359 ribonucleotides. As a result of intramolecular base pairing, a serial arrangement of double-helical sections and internal loops form a unique rod-like secondary structure. PSTV is the first pathogen of a eukaryotic organism for which the complete molecular structure has been established.
The complete nucleotide sequence of citrus exocortis viroid (CEV, propagated in Gymura) and chrysanthemum stunt viroid (CSV, propagated in Cineraria) has been established, using labelling in vilro and direct RNA sequencing methods and a new screening procedure for the rapid selection of suitable RNA fragments from limited digests.The covalently closed circular single-stranded viroid RNAs consist of 371 (CEV) and 354 (CSV) nucleotides, respectively. As previously shown for potato spindle tuber viroid (PSTV, 359 nucleotides), CEV and CSV also contain a long polypurine sequence. Maximal base-pairing of the established CEV and CSV sequences results in an extended rod-like secondary structure similar to that previously established for PSTV and as predicted from detailed physicochemical studies of all these viroids. Although the three viroid species sequenced to date differ in size and nucleotide sequence, there is 60-73 homology between them. As PSTV, CEV and CSV also contain conserved complementary sequences which are separated from each other in the native secondary structure. We postulate that the resulting 'secondary' hairpins, being formed and observed in vitro during the complex process of thermal denaturation of viroid RNA, must have a vital, although yet unknown, function in vivo. The possible origin and function of viroids are discussed on the basis of the characteristic structural features and of a considerable homology with U l a RNA found for a region highly conserved in the three viroids.
When the CCA-halves of tRNAtl" (yeast) and tRNAy' (Escherichiu coli) were incubated with the pG-halves of tRNAYI ( E . coli) and tRNAf' Ia (yeast), respectively, heterologous complexes were detected. When a 10-fold excess of one half was applied, up to 50 of the other half could be complexed. 5 -12 % alanine and valine incorporation was observed into the heterologous combinations in which the pG-halves were derived from tRNAtl" (yeast) and tRNAyaI ( E . coli), respectively. Although the values are small they appear to be significant considering the results of a number of control experiments.The CCA-half of tRNA?'; (yeast) and another fragment of this tRNA which extends from the dihydrouridine region to the CCA-terminus were inactive in the aminoacylation assay but they could be converted into a form which accepted serine under standard conditions even in the absence of a complementary fragment. One activation procedure involved the addition of MgC1, to Mg2+-free fragment solutions, the other consisted in a brief heating-cooling cycle of the fragment solutions at low Mg2+ concentrations. With the two procedures up to 20% or up to 40%, respectively, of the maximal serine incorporation were achieved. At 37 "C the active conformation of the fragments persisted only for a few minutes. Analogously, the CCA-halves of tRNAPhe (yeast), tRNA?" (yeast), and tRNAy' ( E . coli) could be activated although here the extent of aminoacylation varied greatly from one experiment to the other. Mischarging of the activated CCA-halves of tRNAs'; (yeast) and tRNAPhe (yeast) with phenylalanine and serine, respectively, was not observed.The results obtained with the hererologous fragment combinations and with the CCA-halves alone, which at first sight seem to contradict each other, are discussed with respect to the conformational requirements of synthetase-tRNA recognition.The recognition of the tRNAs by their cognate aminoacyl-tRNA synthetases has been the subject of considerable work in the past several years. A number of different methods and approaches have been employed (reviews [l -61). One approach was to investigate the aminoacylation of tRNA molecules from which certain regions have been removed by enzymic or chemical procedures. If the resulting fragment combinations were accepted as substrates by the Abbreviations. tRNA fragments are designated by their terminal nucleotides, c.g. Phe 38 -76 (see also [37]). Fragments obtained by a split in the anticodon of tRNAs are also designated as pG-halves or CCA-halves. Dcqinition. A,, unit, the quantity of material contained in 1 ml of a solution which has an absorbance of 1 at 260 nm. when measured in a I-cm pathlength cell.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.