Processing of the primary transcript for the rat growth hormone gene in vivo.

Abstract

Processing of the rat growth hormone (rGH) gene primary transcript and the effects of thyroid and glucocorticoid hormones on rGH pre-mRNA levels have been studied using subcloned radiolabeled DNA fragments from each of the four introns of this gene as probes. Blot-hybridization analysis of poly(A)+RNA from GC cells, GH3 cells, and normal pituitary gland indicates that processing of intron sequences from the precursor transcript takes place in a qualitatively similar fashion in each of these cell types. The data indicate that, in general, those introns closest to the termini of the primary transcript are removed first followed by removal of the internal introns. The suggested order of removal is IA, ID, IC, and IB. This process is unaffected qualitatively by thyroid or glucocorticoid hormones, both of which increase the rate of transcription of the gene. In addition to the primary transcript and the partially processed intermediate transcripts, GC and GH3 cells were found to contain a heterogenous group of intron-containing polyadenylated rGH gene transcripts which cannot be accounted for by any combination of intron deletions. These transcripts could arise either from internal start sites in the gene, premature termination of transcription, or inefficient processing of rGH mRNA precursors in the transformed cells. Thyroid hormone rapidly increases the levels of intron C-containing transcripts with kinetics that parallel the binding of thyroid hormone receptor to nuclei, but does not alter the ratio of primary to partially processed transcripts. These data suggest that most of the stimulatory activity of this hormone is due to effects on rGH gene transcription and not on pre-mRNA processing.(ABSTRACT TRUNCATED AT 250 WORDS)