Synthesis, cloning, and identification of DNA sequences complementary to mRNAs for alpha and beta subunits of...

Double-stranded cDNA sequences were synthesized, by using as templates mRNA for alpha and beta subunits of thyrotropin purified from mouse thyrotrophic pituitary tumours and cloned in Escherichia coli RR1 by insertion in the Pst I site of the bacterial plasmid pBR322 by use of poly(dA) x poly(dT) homopolymeric extensions. Plasmids containing inserted cDNA sequences were selected by resistance to tetracycline and sensitivity to ampicillin; those containing thyrotropin cDNA sequences were identified by colony hybridization with an 125I-labeled mixture of alpha and beta thyrotropin mRNAs. Plasmids carrying either alpha or beta thyrotropin cDNA sequences were further identified by hybridization to highly purified 125I-labeled alpha or beta thyrotropin mRNA, respectively. Two plasmids, one containing a 400-nucleotide alpha thyrotropin cDNA insert and the other containing a 710-nucleotide beta thyrotropin cDNA insert, were purified and characterized by restriction endonuclease digestions. Plasmid [32P]DNA containing either alpha or beta thyrotropin cDNA was then used as a hybridization probe for further characterization of alpha and beta thyrotropin mRNA from the mouse thyrotropic tumor. RNA was fractionated by agarose gel electrophoresis under denaturing and nondenaturing conditions and transferred to diazobenzyloxymethyl-paper. alpha thyrotropin mRNA of two sizes, 650 and approximately equal to 1500 nucleotides long, were identified. The larger alpha thyrotropin mRNA appeared to have marked secondary structure in its native form in contrast to the 650-nucleotide alpha thyrotropin mRNA. However, only one form of beta thyrotropin mRNA was detected with an apparent size of 710 nucleotides. We have successfully cloned and identified alpha and beta thyrotropin cDNA sequences in bacterial plasmids and used them to identify a second form of alpha thyrotropin mRNA.