ABSTRACT
The acidic, extracellular, glucan endo-1,3-beta-glucosidases (EC 3.2.1.39; beta-1,3-glucanases), pathogenesis-related proteins-2, -N, and -O (i.e. PR-2, PR-N, and PR-O) were purified from Nicotiana tabacum (tobacco) and their partial amino acid sequences determined. Based on these data, complementary DNA (cDNA) clones encoding the proteins were isolated. Additional cDNAs were isolated that encoded proteins approximately 90% identical with PR-2, PR-N, and PR-O. Although the proteins encoded by these cDNAs have not been identified, their deduced amino acid sequences have slightly basic or neutral calculated isoelectric points, as well as carboxy-terminal extensions. These physical characteristics are shared by the vacuolar form of beta-1,3-glucanase and other vacuolar localized analogs of PR proteins, suggesting that the unidentified proteins may be similarly localized. A preliminary evolutionary model that separates the beta-1,3-glucanase gene family from tobacco into at least five distinct subfamilies is proposed. The expression of beta-1,3-glucanase messenger RNAs (mRNAs) in response to infection by tobacco mosaic virus was examined. Messages for the acidic glucanases were induced similarly to the mRNAs for other PR proteins. However, the basic glucanase showed a different response, suggesting that different isoforms are differentially regulated by tobacco mosaic virus infection at the mRNA level.
