Molecular cloning and ethylene-inducible expression of Chib1 chitinase from soybean (Glycine max (L.) Merr.)

A soybean seed-specific PR-8 chitinase, named Chib2, has a markedly extended C-terminal segment compared to other plant Chib1 homologues of the PR-8 chitinase family known to date. To further characterize the molecular structure and the expression pattern of this chitinase family, we cloned two typical Chib1-similar cDNAs (Chib1-1 and Chib1-2) from soybeans by PCR-cloning techniques. The deduced primary sequence of Chib1-1 chitinase is composed of a signal peptide segment (26 amino acid residues) and a mature 273 amino acid sequence (calculated molecular mass 28,794, calculated pI 3.7). This Chib1-1 enzyme is more than 90% identical to Chib1-2 chitinase but is below 50% identical to Chib2 enzyme. Thus, we confirmed the occurrence of two distinct classes, Chib1 and Chib2 in the plant PR-8 chitinase family. The Chib1 genes, interrupted by one intron, were found to be up-regulated in response to ethylene in stems and leaves, but scarcely expressed in developing soybean seeds. Chib1 chitinases may be responsible for protecting the plant body from various pathogenic attacks.

A class III acidic endochitinase is specifically expressed in the developing seeds of soybean (Glycine max [L.] Merr.)

A soybean chitinase which has an apparent molecular mass of 28 kDa by SDS-PAGE, and has chitinase specific activity of 133 units per mg protein at pH 5.2 and an apparent pI of 5.7, was purified from mature dry seeds. Based upon the selected part (the residue positions 10-17) of the determined N-terminal 38 amino acid sequence, a 23-mer degenerate oligonucleotide was synthesized and used for the PCR cloning of the chitinase cDNA. The resulting 1340 bp cDNA was comprised of a 5'-untranslated region of 39 bases, a coding region corresponding to a 25 amino acid signal sequence, followed by a mature 308 amino acid sequence (calculated molecular mass 34,269, calculated pI 4.7), and a 235 nucleotide 3'-terminal untranslated region including 24 bases of the poly(A) tail. By comparing the deduced primary sequence with those of plant chitinases known to date, this enzyme was more than 50% identical to every class III acidic chitinase, but has no significant similarity to other families of chitinases. The comparison also showed that the C-terminal region of this chitinase is markedly extended, by at least 31 residues. Northern blot analysis demonstrated that this mRNA species is remarkably transcribed from the early stage until the late middle stage of seed development, whilst it is hardly expressed in the leaves and the stems of soybean. Spatial and temporal expression of this single gene imply that this class III chitinase is mainly devoted to the seed defense, not only in development but also in dormancy of soybean seed. This is the first reported isolation and cDNA cloning of a class III acidic endochitinase from seeds. According to the chitinase nomenclature we propose that this enzyme would be classified into a new class of chitinase PR-8 family, together with a Sesbania homologue.

A rapid purification method for soybean Bowman-Birk protease inhibitor using hydrophobic-interaction chromatography

A protein fraction was isolated from defatted soybean flour by extraction at acid pH, 40% ammonium sulfate precipitation, hydrophobic interaction chromatography, and gel filtration chromatography. SDS-PAGE, under reducing conditions, confirmed it as a homogeneous preparation. This conclusion was consistent with N-terminal amino acid sequence data (20 cycles) which showed a major sequence identical to those reported for soybean Bowman-Birk-type protease inhibitor (BBI), and also indicated a minimum 95% purity based on recoveries of PTH-amino acid residues. The purified fraction inhibited both trypsin and chymotrypsin with average specific activities of 350 and 672 units mg(-1), respectively. Compared with classical BBI purification, this procedure is very rapid requiring only 72-96 h to achieve a yield of 37 mg purified BBI per 200 g starting material.