Genetic architecture of leaf photosynthesis in rice revealed by different types of reciprocal mapping populations.

The improvement of leaf net photosynthetic rate (An) is a major challenge in enhancing crop productivity. However, the genetic control of An among natural genetic accessions is still poorly understood. The high-yielding indica cultivar Takanari has the highest An of all rice cultivars, 20-30% higher than that of the high-quality japonica cultivar Koshihikari. By using reciprocal backcross inbred lines and chromosome segment substitution lines derived from a cross between Takanari and Koshihikari, we identified three quantitative trait loci (QTLs) where the Takanari alleles enhanced An in plants with a Koshihikari genetic background and five QTLs where the Koshihikari alleles enhanced An in plants with a Takanari genetic background. Two QTLs were expressed in plants with both backgrounds (type I QTL). The expression of other QTLs depended strongly on genetic background (type II QTL). These beneficial alleles increased stomatal conductance, the initial slope of An versus intercellular CO2 concentration, or An at CO2 saturation. Pyramiding of these alleles consistently increased An. Some alleles positively affected biomass production and grain yield. These alleles associated with photosynthesis and yield can be a valuable tool in rice breeding programs via DNA marker-assisted selection.

Catalytic mechanism of beta-amylase from Bacillus cereus var. mycoides: chemical rescue of hydrolytic activity for a catalytic site mutant (Glu367-->Ala) by azide.

The hydrolytic activity of beta-amylase from Bacillus cereus var. mycoides was lost on replacement of either of the catalytic residues (Glu172 or Glu367) with an alanyl residue. When maltopentaose and 2 M azide existed together mutant, E367A cleaved the glucosidic linkage of maltopentaose and produced maltose at pH 7.0 and 25 degrees C, but the other mutants (E172A and double mutant E172A/E367A) did not. This indicates that azide acts as a general base instead of E367 and Glu172 acting as general acids, and that the hydroxide ion generated from a water molecule activated by azide attacks a reactive pyranose nucleophilically so that beta-maltose is produced.