ANTISENSE EXPRESSION OF CHAPERONIN 60-BETA IN TRANSGENIC TOBACCO PLANTS LEADS TO ABNORMAL PHENOTYPES AND ALTERED DISTRIBUTION OF PHOTOASSIMILATES

Chaperonins are a class of molecular chaperone, present in bacteria, m itochondria and chloroplasts, that are involved in protein folding and assembly in many organisms. Plastid alpha and beta chaperonins have b een suggested to be involved specifically in the assembly of Ribulose bisphosphate carboxylase/oxygenase. However, to date there is no direc t evidence to confirm the in vivo role of plastid chaperonin 60 polype ptides as molecular chaperones. This paper reports on the production, by means of antisense technology, of transgenic tobacco plants with re duced levels of chaperonin 60 beta (Cpn60 beta). Antisense cpn60 beta plants showed drastic phenotypic alterations including slow growth, de layed flowering, stunting and leaf chlorosis. The most extreme effect appeared to be lethality suggesting that cpn60 beta functions are esse ntial for viability. Cpn60 beta antisense plants accumulated Rubisco t o specific activities equal to or higher than that of controls and had high plastid starch contents. These observations are discussed with r espect to the suggestion that chaperonin is required for the assembly of active Rubisco in vivo. In addition, metabolic alterations in the a ntisense transgenic plants such as reduced soluble carbohydrate conten t as well as higher levels of starch in chloroplasts, suggest that Cpn 60 beta may be required for import, assembly or membrane insertion of several chloroplast membrane proteins. These results are in agreements with the proposed role of Cpn60 beta as a molecular chaperone.