A Brassica napus gene family which shows sequence similarity to ascorbate oxidase is expressed in developing pollen. Molecular characterization and analysis of promoter activity in transgenic tobacco plants

The genomic clone named Bp10 contains a member of a small pollen-specific gene family of B. napus. The expression of the Bp10 gene family is maximal in early binucleate microspores and declines considerably in mature trinucleate pollen. Homologues of the Bp10 genes are expressed in the pollen of other plant species. The pollen-specific expression of the gene contained in the genomic clone was confirmed in tobacco plants transformed with a chimeric Bp10 promoter/GUS construct. A promoter fragment of 396 bp is sufficient to direct a strong and correct spatial and temporal expression in transgenic plants. The Bp10 gene family codes for proteins of 62 kDa showing approximately 30% sequence identify to cucumber and pumpkin ascorbate oxidases (AAOs). However, the AAO active centres are not conserved in the Bp10 products, suggesting an evolutionary relationship but a different enzymatic activity for these proteins. Expression of a recombinant Bp10 protein in E. coli inhibits bacterial growth on minimal medium, suggesting the production of an enzymatically active polypeptide in bacteria. No AAO activity could be correlated with the expression of the recombinant protein. Moreover, substances affecting AAO activity do not appear to influence the inhibitory activity of the protein produced in bacteria. However, as indicated by the rescue of bacterial growth in the presence of sodium bicarbonate or gaseous CO2, the Bp10 protein activity could be modulated by CO2 levels.