Evidence for the synthesis of a precursor of carp proinsulin in a cell free translation system

Abstract

Until recently the cell free translation of proinsulin mRNA* has not been successful. This is due to the difficulties in obtaining sufficient amounts of endocrine tissue of the pancreas and to the high content of ribonuclease. The islets of Iangerhans of fish (Brockmann bodies) are very suitable objects. They can be isolated with only little contamination by exocrine tissue and are much bigger than those of mammals. In previous experiments we characterized the islets of carp (C~@nus carpio) by their ability to synthesize proinsulin in tissue culture [ 1 ] and by their content of insulin (unpublished results). The synthesis of a proinsulin-like material could also be shown using polysomes of the carp islets in a rat liver cell free system [2]. While the work was in progress two relevant papers appeared. Yip et al. reported the translation of total RNA from rat islets and tumor tissue in the cellular system of oocytes [3]. Lomedico and Saunders used a wheat germ cell free system for the translation of poly A’-RNA from adult dog or fetal bovine pancreas [4]. Whereas in both cases products larger than proinsulin appeared to be synthesized there was a discrepancy with respect to the molecular weights (18 000 and 12-l 3 000 dalton, respectively). The present paper deals with the cell free translation in a wheat germ system of poly A’-RNA from