Probable nuclear precursors of preprolactin mRNA in rat pituitary cells

In the rat pituitary, several species of preprolactin (pPrl) mRNA larger than the mature mRNA have been reported (Maurer, R.A., Gubbins, E.J., Erwin, C.R., and Donelson, J.E. (1980) J. Biol. Chem. 255, 2243-2246). We examined the size distribution of pPrl transcripts in nuclear, cytoplasmic, and nuclear poly(A)-containing RNA from rat pituitary tumor MtTW10 and from the pituitaries of diethylstilbestrol-treated Fischer 344 x Holtzman rats. RNA was electrophoresed in agarose/2.2 M formaldehyde horizontal slab gels, transferred to diazobenzyloxymethyl paper or to nitrocellulose paper, and hybridized with a nick-translated recombinant DNA probe containing rat pPrl cDNA sequences. Up to 8 putative precursor mRNA molecules were detected which were from 2 to 13 times the length of mature pPrl mRNA. Three of these RNA species were larger than the maximum size of putative precursors previously reported. These probable precursors were nuclear in origin and polyadenylated, at least in the case of pituitary tumor cells. The largest molecule, 14 kilobase pairs long, may approximate the size of the initial transcript of the prolactin gene.