Soybean membranes possess high‐affinity binding sites for fungal β‐glucans that elicit phytoalexin synthesis. The ability of 1,3‐1,6‐β‐glucans, released by acid hydrolysis from mycelial walls of Phytophthora megasperma f.sp. glycinea, to compete for the putative phytoalexin elicitor receptors increases with their average degree of polymerization (DP). The results suggest a function where the probability for glucan fragments of containing a structural determinant that is optimal for binding approaches 1 as the DP tends to infinity. Ligand displacement data obtained against a 125I‐labeled glucan elicitor (average DP= 18) provided a theoretical minimum IC50 (50% inhibitory concentration) for 1,3‐1,6‐β‐glucans of 3 nM. The IC50 value obtained for a synthetic hepta‐β‐glucoside having a known elicitor‐active structure was 8 nM, remarkably close to the predicted value. Displacement of the 125I‐glucan of large DP was uniform and complete showing that the heptaglucoside had access, with similar affinity, to all sites available to the radioligand. Further analysis using a 125I‐labeled aminophenethylamine derivative of the heptaglucoside suggested that the putative glucan‐elicitor receptors bind a basic structural determinant present in all elicitor‐active glucans from the soybean pathogen P. megasperma.

The expression of the gene for variant surface glycoprotein (VSG) 118 in Trypanosoma brucei is activated by transposing a DNA segment containing the gene and 1-2 kb in front of it to an expression site elsewhere in the genome. By S1 nuclease protection and RNA blotting experiments we show here the presence of several minor transcripts in trypanosomes synthesizing VSG 118, one of which covers the entire transposed segment. Comparison of the sequence of the 5' terminal segment of VSG 118 messenger RNA (mRNA), determined by primed reverse transcription, and the corresponding region of the 118 VSG gene, shows that the 5' terminal 34 nucleotides of the mRNA are not encoded in the 118 VSG gene contiguous with the remainder of the mRNA. We conclude that synthesis of a VSG mRNA involves splicing of a much longer primary transcript, which may start outside the transposed segment.

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Hairpin structures formed by seven DNA inverted repeats have been studied by PAGE, UV(CD)-spectroscopy and nuclease cleavage. The hairpins consisted of (CG)3 stems and loops of 2, 3 and 4 residues. Thermal stabilities (Tm) have been determined in low and high ionic strength buffers, where the hairpins were structured in the B- and Z-DNA form respectively. The thermodynamic parameters of hairpin formation have been obtained by a two-state analysis of the hairpin-coil transitions. It is found that, on increasing the number of bases in the loop from 2 to 3 and 4, the Tms of the B-hairpins decrease, whereas the Tms of the same hairpins in the Z-form increase. This confirms previous evidence (1,2) that in a hairpin molecule the size and structure of the loop are modulated by the conformation of the helical stem. Moreover, B-hairpins with loops comprising 2, 3 and 4 bases have been digested with the single-strand-specific nuclease from mung bean. In our experimental conditions (0 degrees C) the nuclease preferentially cleaves the unbonded nucleotides of the loops. However, the rates of loop hydrolysis, which roughly follow a first-order kinetics, markedly depend on the size of the loop. At a ratio of 3 enzyme units/micrograms DNA, the half-lives of hairpins which are expected to form loops of 4, 3 and 2 residues are 90, 145 and 440 minutes respectively. Thermostability and enzymatic digestion data suggest that two-membered loops can be formed in B-hairpins but not in Z-hairpins.