In beta-glucans those beta-1,4 glycosidic bonds which are adjacent to beta-1,3 bonds are cleaved by endo-1,3-1,4-beta-glucanases (beta-glucanases). Here, the relationship between structure and activity of the beta-glucanase of Bacillus macerans is studied by x-ray crystallography and site-directed mutagenesis of active site residues. Crystal structure analysis at 2.3-A resolution reveals a jelly-roll protein structure with a deep active site channel harboring the amino acid residues Trp101, Glu103, Asp105, and Glu107 as in the hybrid Bacillus beta-glucanase H(A16-M) (Keitel, T., Simon, O., Borriss, R., and Heinemann, U. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 5287-5291). Different mutant proteins with substitutions in these residues are generated by site-directed mutagenesis, isolated, and characterized. Compared with the wild-type enzyme their activity is reduced to less than 1%. Several mutants with isosteric substitutions in Glu103 and Glu107 are completely inactive, suggesting a direct role of these residues in glycosyl bond hydrolysis. The kinetic properties of mutant beta-glucanases and the crystal structure of the wild-type enzyme are consistent with a mechanism where Glu103 and Glu107 are the catalytic amino acid residues responsible for cleavage of the beta-1,4 glycosidic bond within the substrate molecule.

Hybrid (1-3,1-4)-beta-glucanase genes were constructed by extension of overlapping segments of the (1-3,1-4)-beta-glucanase genes from Bacillus amyloliquefaciens and B. macerans generated by the polymerase chain reaction (PCR). Four hybrid genes were expressed in Escherichia coli cells. The mature hybrid enzymes contain a 16, 36, 78, or 152 amino acid N-terminal sequence derived from B. amyloliquefaciens (1-3,1-4)-beta-glucanase followed by a C-terminal segment derived from B. macerans (1-3,1-4)-beta-glucanase. Biochemical characterization of parental and hybrid enzymes shows a significant increase in thermostability of three of the hybrid enzymes when exposed to an acidic environment thus combining two important enzyme characteristics within the same molecule. At pH 4.1, 85%-95% of the initial activity was retained after 1 h at 65 degrees C in contrast to 5% and 0% for the parental enzymes from B. amyloliquefaciens and B. macerans. After 60 min incubation at 70 degrees C, pH 6.0, the parental enzymes retained 5% or less of the initial activity whilst one of the hybrids still exhibited 90% of the initial activity. Of the parental enzymes B. macerans (1-3,1-4)-beta-glucanase had the lower specific activity while the hybrid enzymes exhibited specific activities that were 1.5- to 3-fold higher. These experimental results demonstrate that exchange of homologous gene segments from different species may be a useful technique for obtaining new and improved versions of biologically active proteins.

The codon usage of a hybrid bacterial gene encoding a thermostable (1,3-1,4)-13-glucanase was modified to match that of the barley (1,3-1,4)-p-glucanase isoenzyme (4) and their degradation is a prerequisite for the enzymatic mobilization of endosperm storage components, which serve as nutrients for the growing embryo. Efficient degradation of endosperm cell walls is also important for utilization of barley as a monogastric animal feed (5, 6) and in industrial processes such as malting and brewing (7). Furthermore, extraction of non-food products deposited in the endosperm of transgenic barley would be facilitated by the action of highly efficient, heat stable cell wall-degrading enzymes. The (1,3-1,4)-3-glucanases (EC 3.2

A small cysteine-rich protein, the function of which remains elusive, was discovered in the exudate of a Penicillium species. Crystal diffraction experiments conducted using in-house Cu Kalpha radiation and an R-AXIS IV++ imaging-plate detector yielded high-quality data to 1.4 A, with a distinguishable anomalous signal from sulfur (DeltaF/F = 0.031). This was used to phase the data and solve the structure using a single data set; the 64-residue amino-acid sequence was unambiguously determined from the electron density. It revealed a globular all-beta protein with a hitherto unknown fold, having a surface electrostatic charge distribution that is similar to that of another small secreted fungal protein, the Williopsis mrakii killer toxin. Aligning the charge distribution superimposed the potential recognition sites of the two proteins, suggesting a similar negatively charged target.

The molecular basis for the absence of anthocyanins and proanthocyanidins in four independent sodium azide-induced antl8 mutants of barley was examined by sequencing the gene encoding dihydroflavonol 4-reductase in these mutants. Sodium azide generated 21 base substitutions, which corresponds to 0.17% of the 12,704 nucleotides sequenced. Of the substitutions, 86% were nucleotide transitions, and 14% were transversions. AT -G-C base pair transitions were about 3 times more frequent than G-C -) AT transitions. No deletions or mutation hot spots were found. The absence of dihydroflavonol 4-reductase activity in antl8-159, antl8-162, and antl8-164 plants is caused by missense mutations in the respective genes. By using microprojectile bombardment, a plasmid harboring the wild-type Antl8 gene was introduced into antl8-161 mutant cells and resulted in the development of anthocyanin pigmentation, which demonstrates that the mutation is corrected by expression of the introduced gene. On the other hand, a plasmid derivative with the two antl8-161-specific base transitions at the 5' splice site of intron 3 prevented complementation. It is concluded that the absence of detectable mRNA for dihydroflavonol 4-reductase in antl8-161 cells is due to the mutations in the pre-mRNA splice donor site.