A Second Thyroglobulin Messenger RNA Species (rTg-2) in Rat Thyrocytes

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Abstract
A 0.95-kilobase (kb) thyrocyte RNA, initially detected in our 1B-6 subclone of Fisher rat thyrocytes (FRTL-5) using an oligonucleotide probe complementary to the 5'' end of rat thyroglobulin (Tg) mRNA, was also detected in cultured thyrocytes of the Wistar rat (WRT cells) and in freshly isolated normal rat thyroid tissue. This transcript was thyroid specific and as abundant as the previously characterized 9.0-kb Tg mRNA in the cultured thyroid cells under the growth conditions employed. The smaller RNA (designated rTg-2 mRNA) was cytoplasmic, polyadenylated, and regulated by TSH. Preliminary characterization with several oligonucleotide probes showed that rTg-2 shared coding information present at the 5'',but not the 3'', end of the 9.0-kb Tg mRNA. Sequencing of the cloned rTg-2 cDNA showed that it was homologous to human and other higher vertebrate Tg cDNAs at its 5'' coding end, but contained additional nonhomologous coding and noncoding sequences at the 3'' end. The junction between the shared and unique sequences in rTg-2 mRNA occurred at the exon-5/intron-5 boundary in the Tg gene. This smaller transcript has not previously been reported and encodes elements known to be of structural and functional significance in the 330-kD Tg monomer. A putative polypeptide product from rTg-2 mRNA may play an important role in thyroid function and thyroid autoimmunity.