fur Biochemie9 BayerischeA new modification of silver staining is presented which utilizes two chemical properties of thiosulfate: image enhancement by pretreatment of fixed gels, and formation of soluble silver complexes which prevents unspecific background staining during image development. This procedure provides high sensitivity for proteins, RNA and DNA in the nanogram range on a colorless, transparent background. The performance of this method is documented by staining one-and two-dimensional patterns of plant leaf proteins. Moreover, we achieved, for the first time, the detection of the non-structural, tobacco mosaic virus-specific 126 kDa protein directly in the onedimensional protein pattern of infected protoplasts by a staining procedure.Julius-Maximilians-Universitat, Wurzburg

Viroids are uncoated infectious RNA molecules pathogenic to certain higher plants. Four different highly purified viroids were studied. By ultracentrifugation, thermal denaturation, electron microscopy, and end group analysis the following features were established: (i) the molecular weight of cucumber pale fruit viroid from tomato is 110,000, of citrus exocortis viroid from Gynura 119,000, of citrus exocortis viroid from tomato 119,000, and of potato spindle tuber viroid from tomato 127,000. (ii) Viroids are single-stranded molecules. (iii) Viroids exhibit high thermal stability, cooperativity, and selfcomplementarity resulting in a rod-like native structure. (iv) Viroids are covalently closed circular RNA molecules. Viroids exert biological functions like viruses since they are infectious and pathogenic. In contrast to viruses, however, all known viroids seem to exist only as uncoated RNA molecules and their genetic information is not sufficient to code for a protein with a molecular weight larger than 10,000. They are pathogens of certain higher plants. Little is understood, as yet, about their mechanisms of replication and pathogenicity. Moreover, the available information is too limited to lead to a concise structural model of viroids (1). Previous data on molecular weights of viroids were obtained by indirect methods and the determinations were based on particular assumptions about their unknown conformation. Analysis in sucrose gradients, in polyacrylamide gels, and in the electron microscope resulted in molecular weight values ranging between 25,000 and 150,000 for both the viroid of the spindle tuber disease of potato (PSTV) (e.g. refs. 2-4) and the viroid of the exocortis disease of citrus (CEV) (e.g., refs. 5-7). The sedimentation coefficients reported range from 4 S to 25 S (2, 6, 7). A considerable number of structural models have been proposed (e.g. refs. 4, 7, and 8); single strands with hairpins or double strands with incomplete base pairing have been considered as the most probable structure of viroids.The availability of highly purified viroid RNA in sufficient quantities enabled us to investigate the structure in detail.

No abstract

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.

Eukaryotic tRNAs are synthesized in the nucleus and need to be exported to the cytoplasm where they function in translation. tRNA export is mediated by exportin-t, which binds tRNA directly and with high affinity. tRNAs are initially synthesized as precursor molecules. Maturation to functional tRNA takes place in the nucleus, precedes export, and includes trimming of the 59 and 39 ends, posttranscriptional addition of the 39 CCA end, nucleoside modifications, and in some cases splicing. Here we address the question of how tRNA maturation is coordinated with export and thus how cytoplasmic accumulation of inactive maturation intermediates is avoided. This could, in principle, be achieved by nuclear retention of immature tRNA or by selective export of the fully mature form. We show that exportin-t has a strong preference for tRNA with correctly processed 59 and 39 ends and nucleoside modification. tRNA recognition by exportin-t can thus be considered as a quality control mechanism for these maturation steps prior to tRNA export. Surprisingly however, exportin-t can efficiently bind unspliced tRNA and intron-containing tRNA is exported when the rate of splicing is slow. During characterization of the exportin-t /tRNA interaction we found that exportin-t recognizes features in the tRNA that are conserved between prokaryotic and eukaryotic tRNAs. Our data suggest that correct tRNA shape, the 59 and 39 terminal ends, and the TCC loop are critical for exportin-t binding.

Although the numerical abilities of many vertebrate species have been investigated in the scientific literature, there are few convincing accounts of invertebrate numerical competence. Honeybees, Apis mellifera, by virtue of their other impressive cognitive feats, are a prime candidate for investigations of this nature. We therefore used the well-established delayed match-to-sample paradigm, to test the limits of honeybees' ability to match two visual patterns solely on the basis of the shared number of elements in the two patterns. Using a y-maze, we found that bees can not only differentiate between patterns containing two and three elements, but can also use this prior knowledge to differentiate three from four, without any additional training. However, bees trained on the two versus three task could not distinguish between higher numbers, such as four versus five, four versus six, or five versus six. Control experiments confirmed that the bees were not using cues such as the colour of the exact configuration of the visual elements, the combined area or edge length of the elements, or illusory contours formed by the elements. To our knowledge, this is the first report of number-based visual generalisation by an invertebrate.

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.

Extracts of wheat germ contain an RNA ligase activity that catalyses the conversation of linear polyribonucleotides into covalently closed circles. The newly formed 3',5'-phosphodiester bridge is resistant to alkali and a number of ribonucleases. This unusual stability is due to the presence of a phosphoryl group esterified to the 2'-position of the 3'-nucleotide joined. This is the first demonstration of an RNA ligase in higher plants and of a natural 2'-phosphomonoester, 3',5'-phosphodiester linkage.