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RNA is involved in many biological functions, ranging from information storage and transfer to the catalysis of reactions involving both nucleic acids and proteins. Previous crystallographic studies on RNA oligomeric chains provide only averaged structures or information limited in resolution. The oligomer [U(U-A)6A]2 was chosen for the study of protein-RNA interactions in viruses. Its size and base composition mimic portions of the genomic RNA in alfalfa mosaic virus that bind to the amino terminus of the viral subunit. The actual sequence was designed to guarantee the formation of a single species of duplex and to facilitate the production of the pure oligomer in large quantities. The molecular structure, derived from the 2.25 A resolution X-ray diffraction data, allows the most detailed analysis of an A-RNA helix reported to date. Two kinks are observed that divide the duplex into three blocks, each close to a canonical A-helical conformation. A few intermolecular hydrogen bonds involving 2'-hydroxyl groups stabilize this peculiar conformation of the RNA, which may be related to the temperature used for the crystallization (35 degrees C). The structure demonstrates both the plasticity of the RNA molecule and the role of the 2'-hydroxyl groups in intermolecular interactions.

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Both polarities ofthe satellite RNA of tobacco ringspot virus are sources of self-cleaving sequences. RNA of the less abundant, negative polarity, designated sTobRV-(-)RNA, has cleaving activity that was mapped previously to two noncontiguous regions of the polyribonucleotide chain.Endoribonucleolytic oligoribonucleotides (E) corresponding to the larger of the two regions cleaved smaller substrate oligoribonucleotides, at the ApG phosphodiester that is cleaved in sTobRV(-)RNA. An analogue of the substrate, which has a 2'-5' ApG phosphodiester, was not cleaved by E but acted as a competitive inhibitor of the cleavage of substrate. The analogue served as a primer, and E served as template, for reverse transcriptase-catalyzed copying of specific E sequences. The sequences transcribed suggest base pairing between the 5' region of E and a portion of the substrate that is located 3' to, but does not include, the ApG phosphodiester. Results from other experiments indicate this base pairing is a part of the functional cleavage complex. The .aation of the ends of E and substrate anticipates a second, 4-base-pair association between E and a portion of substrate that is 5' to, but does not include, the ApG phosphodiester. The effects of compensating mutations in E and substrate oligoribonucleotides support the existence of this second association in the active cleavage complex.Several RNA molecules undergo autolytic cleavage to generate a 2',3'-cyclic phosphodiester and a 5'-hydroxyl as the new chain end groups. Most of these self-cleaving RNA sequences are derived from small satellite RNAs associated with plant viruses of the nepovirus and sobemovirus groups (1, 2), and most conform to a consensus secondary structure. termed a "hammerhead structure," which also applies to a self-cleaving viroid RNA (2, 3). The form ofthe satellite RNA of tobacco ringspot that is most abundantly encapsidated has a self-cleaving sequence that conforms to the hammerhead structure, whereas the self-cleaving complement of this RNA, designated sTobRV(-)RNA, does not have the conserved sequences of the consensus secondary structure.

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