The availability of (yeast)tRNAPhe for helix formation with complementary oligoribonucleotides has been explored by equilibrium dialysis studies. Oligoribonucleotides complementary to sequences which are double-stranded, according to the cloverleaf model of tRNAPhe, did not bind. Two regions of tRNAPhe were fully acessible to oligonucleotide binding: (a) the 3'-terminus; (b) the sequence (3'-5')U-G-A-A-Y (33-37) of the anticodon loop. Two other regions were accessible to oligonucleotide binding: (a) part of the extra arm; (b) the (3'-5')T-Y-C sequence of the ribosylthymine loop. However, the binding constants of trimers and tetramers complementary to these parts of tRNAPhe are very low.The data indicate that the anticodon loop of tRNAPhe has an asymmetric structwe, which is not compatible with the Fuller-Hodgson model. The low binding constants for oligonucleotides complementary to the extra arm or to the TpVpC sequence are discussed. They seem to be due either to sterical hindrance or to different conformations of tRNA. I n the latter case, one conformation would be accessible to complementary oligonucleotides for helix formation, the other would not. Present available oligonucleotide binding data of four tRNAs suggest that each tRNA has an individual pairing scheme, according to which the loops of the cloverleaf contribute to the three-dimensional structure.Although many properties of tRNA have been clarified, the three-dimensional structure of tRNA is far from known. The results of a variety of physicochemical measurements have led to ample evidence that tRNA has a structure of higher order than a loose combination of helical segments as is proposed by the general cloverleaf model [I-31. Several models of the tertiary structure of tRNA have recently been proposed in order to obtain a correlation between molecular structure and biological function of this molecule. However, none of them is apparently consistent with all the available data, which are pertinent for the actual three-dimensional structure of tRNA (for review, see [I]). This lack in understanding the structure of tRNA is mainly due DefiniGon. Azao unit; the quantity of material contained in 1 ml of a solution which has an absorbance of 1 a t 260 nm when measured in a 1-cm path-length cell.to the fact that it is not known what segments or parts of the loops of the cloverleaf are involved in the organization of the higher ordered structure of tRNA.Recently, it has been shown that it is possible to explore, in partially double-stranded RNA molecules, single-stranded regions by complementary oligonucleotide binding [4 -81. Oligonucleotides of three units interact with available complementary regions of RNA to such a degree that the association constant can be readily measured by equilibrium dialysis. We applied this methodology to see what regions of (yeast) tRNAPhe are open, unshielded and unpaired and are thus available for oligonucleotide binding. It is apparent that this method can only be used to detect sequences, which are not shorter than three ...

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The full-length cDNA sequences of three variable surface glycoproteins from bloodstream forms of Trypanosoma congolense have been determined. They encode preproteins of 429, 449, and 428 amino acids. These proteins contain the typical N-terminal leader sequences of secreted eukaryotic proteins, and display hydrophobic amino acids at their C-termini characteristic of variable surface glycoproteins ; these leader sequences serve as transient membrane anchors after protein synthesis. By performing sequence comparisons of all currently known variable surface glycoproteins from 7: congolense, several conserved elements could be identified. These elements included positional conservation of most of the cysteine residues, conservation of the flanking sequences surrounding these cysteine residues, clustering of proline residues near the C-termini, and a hydrophobic heptad motif near the end of the N-terminal domains. The N-terminal domains seem to be closely related to the B domains of Trypanosoma brucei variable surface glycoproteins, whereas the C domains have up to now only been identified in 7: congolense variable surface glycoproteins. The data suggest that 7: congolense variable surface glycoproteins, despite low sequence similarities, could have conserved tertiary structures.African trypanosomes evade the immune system of their mammalian host by antigenic variation caused by rapidly switching the expression of the major surface glycoprotein or variable surface glycoprotein (VSG) genes [l]. Each organism has approximately one thousand different VSG genes [2], many of which can be expressed [3]. Usually, however, only one VSG gene is expressed at a time.VSG molecules form a homogeneous monolayer on the surface of bloodstream trypanosomes protecting underlying plasma membrane constituents from the humoral immune response. The variable antigenic determinants do not interfere with VSG packing into the protecting surface coat. Therefore, it is assumed that common structural features exist in antigenically differing VSGs that should be detectable in the DNA or protein sequences.To date, approximately 20-30 complete or partial amino acid sequences are known for antigenically distinct VSG from the Trypanosoma brucei group [4 -61. From Trypanosoma congolense only two complete amino acid sequences have been described for bloodstream VSG [7], and two additional sequences have been described for metacyclic VSG [8]. All VSG display N-terminal signal peptides and C-termi- Note. The novel sequence data published here have been submitted to the EMBL sequence data bank(s) and are available under accession number(s) X79399, X79400, X79401 for BENat 1-VSG, BENat 1.2-VSG and BENat 1.3-BSG mRNAs, respectively. nal hydrophobic amino acids which are absent in the mature proteins. Based primarily on conserved cysteine motifs, I ? brucei VSG have been grouped into several subfamilies [6]. The two 7: congolense VSG included in this analysis resemble N-terminal domain type B of 7: brucei VSG. The sequence similarity of VSG is low, even betw...