IDENTIFICATION OF SEPARATE MESSENGER-RNAS CODING FOR THE ALPHA-SUBUNITS AND BETA-SUBUNITS OF THYROTROPIN

A 9SmRNA, purified from mouse thyrotropic pituitary tumors by sucrose density gradient centrifugation of poly(A)-enriched mRNA, directed the synthesis of only α and β subunits of thyrotropin in the reticulocyte lysat translation system. Analysis of radioiodinated 9S mRNA, repurified by oligo(dT)-cellulose chromatography and sucrose gradient centrifugation, yielded two species of RNA on urea/polyacrylamide gel electrophoresis. The major RNA species contained 620 nucleotides, and the minor RNA species contained 560 nucleotides. Unlabeled 9S mRNA was further purified by urea/polyacrylamide gel electrophoresis; the mRNAs were separately eluted from slices of the gel containing material migrating with an apparent length of 620 and 560 nucleotides. Translation of these mRNAs in the reticulocyte lysate showed that the longer mRNA coded for the α subunit and the shorter mRNA coded for the β subunit of mouse thyrotropin. Because more α than β subunit of thyrotropin was consistently synthesized, unbalanced amounts of thyrotropin subunits appear to be synthesized by translation of unbalanced amounts of individual mRNAs. We have demonstrated that the synthesis of thyrotropin is directed by two separate mRNA molecules, each coding for a different subunit of the hormone.