Synthesis of pathogenesis-related proteins in tobacco is regulated at the level of mRNA accumulation and occurs on membrane-bound polysomes.

The pathogenesis-related (PR) proteins of tobacco plants are induced in response to a variety of pathogenic and chemical agents. Although the function of these proteins is unknown, they are associated with resistance to multiplication and/or spread of tobacco mosaic virus. We report that functional mRNAs encoding PR proteins are present only when synthesis of these proteins has been induced, suggesting that their synthesis is controlled in part at the level of mRNA accumulation. In addition PR proteins appear to be synthesized and processed in a manner analogous to proteins destined for the endoplasmic reticulum since (i) the in vitro translation products synthesized in the wheat-germ cell-free system are slightly larger than the in vivo products, (ii) translation of PR mRNAs in the rabbit reticulocyte lysate system is blocked unless that system is supplemented with dog pancreas microsomes, and (iii) mRNAs for PR proteins are associated predominantly with membrane-bound polysomes in vivo. This pathway of synthesis and posttranslational modification suggests possible sites of action of these proteins.