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

The splicing of U12-type introns can be a rate-limiting step in gene expression

Some protein-coding genes in metazoan genomes contain a minor class of introns that are excised by a distinct, low-abundance spliceosome. We have developed a quantitative RT-PCR assay that allows comparison of the relative rates of intron removal from the transcripts present in a pre-mRNA population. We show that the U12-type introns are more slowly spliced than the major-class (U2-type) introns from three endogenous pre-mRNAs in human tissue culture cells. In Drosophila melanogaster S2 cells, using minigene constructs designed to produce nearly identical mRNAs, we observe increased expression of fluorescent protein and mature mRNA upon mutation of a U12-type to a U2-type intron. These results provide evidence that the level of gene expression in vivo is lowered by the presence of a U12-type intron and implicate the U12-type spliceosome as a target in the post-transcriptional regulation of gene expression.

Ultrastructural distribution of growth hormone (GH) mRNA and GH intron I sequences in rat pituitary gland: effects of GH releasing factor and somatostatin

We have measured the distribution of growth hormone (GH) mRNA or intron I sequences by in situ hybridization on ultrathin frozen sections of pituitaries removed from rats injected with saline, GH-releasing factor (GRF) or somatostatin. A 4-fold increase in labeling of the anterior lobe was observed after GRF, no changes with somatostatin. After ultrastructural in situ hybridization, labeling with the GH cDNA probe was specific to somatotrophs. Two populations of cells containing few or many secretory granules were labeled mainly in the cytoplasm or in both cytoplasm and nucleus. Some cells showed labeling at the perinuclear membrane. Hybridization with the GH intron I probe showed the same cell specificity with silver grains mainly located in the nucleus. After GRF, sequences hybridizing to growth hormone cDNA were increased mainly in the nucleus of somatotrophs when compared to mock-injected rats, indirectly suggesting an increase in the transcriptional activity of the growth hormone gene. After somatostatin, the density of labeling in the nucleus was increased suggesting that somatostatin may prevent the export of growth hormone mRNA molecules from the nucleus to the cytoplasm.