Isolation of catfish proinsulin messenger RNA and synthesis of its complementary DNA

The purpose of these experiments was to obtain proinsulin mRNA from catfish pancreatic islets and synthesize its cDNA. Poly(A)-rich mRNA was electrophoresed on preparative agarose-urea gels. One RNA fraction was obtained which translated predominantly preproinsulin. This mRNA was estimated to be approx. 210 000 Mr (650 nucleotides) when electrophoresed under denaturing conditions. [3H]Proinsulin cDNA was hybridized to excess RNA to monitor purification of mRNA from total islet RNA. Greater than 94% of proinsulin messenger contained poly(A) sequences. [3H]Proinsulin cDNA hybridized to its template mRNA with a Rot 1/2 of 4.4 x 10(-3) mole x sec/l. The overall purification was 80-fold by this type of analysis. Thermal denaturation studies indicated a high degree of fidelity of hybrid formation between [3H]proinsulin cDNA and proinsulin mRNA. Proinsulin comprised 20% of total islet protein when synthesis was measured in vivo (Albert and Permutt, 1979) and 12-20% when total islet mRNA was translated in a cell-free system. Using the [3H]proinsulin cDNA probe it was estimated that proinsulin mRNA accounted for approx. 15% of total islet mRNA.