In vitro processing of the human growth hormone primary transcript

Splicing variants of the human growth hormone mRNA: detection in pituitary, mononuclear cells and dermal fibroblasts

The human growth hormone/human chorionic somatomammotropin (hGH/hCS) gene cluster contains five genes: hGH-N, hGH-V, hCS-B, and hCS-L. In this study, the nature of splicing products of their primary transcripts (except hCH-L) was analyzed by nuclease mapping as well as by reverse transcription-polymerase chain reaction (RT-PCR) experiments All the previously described hGH-N mRNAs encoding the normal 22-K growth hormone, the 20-K variant as well as a transcript lacking the third exon were found in pituitary tissue and pituitary tissue and in transiently transfected human 293-S cells. In addition, splicing products lacking either exons 3 and 4 exons 2,3 and 4 were found in both tissues. In accordance to previously reported data, the hGH-V, the hCS-A and the hCS-B genes which are expressed in placental tissue give rise to the 22-K mRNA but not to 20-K mRNA. Furthermore, no hCS mRNA arising from skipping of exon 3 was present, whereas mRNAs arising from ligation of exon 2 to exon 5 and of exon 1 to exon 5 were clearly detectable. The various hGH cDNas were expressed in vivo and screened for lactogenic activity. Only the 22-K and the 20-K variant were active in this assay. All of the hGH-N-derived differentially processed RNAs were found in cell lines of lymphoid (Hut-78) and of myelomonocytic type (U937), which had been recently described to secrete growth hormone. Interestingly, RT-PCR analysis allowed the determination of hGH-N transcripts in dermal fibroblasts. This finding underlines the importance of growth hormone in influencing immune system development and further suggests possible autocrine/paracrine regulatory loops in skin tissue.

Purification of two picornaviral 2A proteinases: interaction with eIF-4 gamma and influence on in vitro translation

A mammalian cell infected with a human rhinovirus or enterovirus has a much reduced capability to translate capped mRNAs (the host cell shutoff), while still allowing translation of uncapped viral RNA. Biochemical and genetic evidence suggests that the viral proteinase 2A induces cleavage of the eukaryotic initiation factor (eIF) 4 gamma (also known as p220) component of eIF-4 (formerly called eIF-4F). However, neither the mechanism underlying the specific proteolysis of eIF-4 gamma nor the influence of this cleavage on the translation of capped mRNAs has been clarified. Such studies have been hampered by a lack of large quantities of a purified 2A proteinase. Therefore, the mature proteinases 2A of human rhinovirus 2 and coxsackievirus B4 were expressed in soluble form in Escherichia coli. A four-step purification protocol was developed; 1 mg of highly purified 2A proteinase per gram wet weight of E. coli was obtained. Both enzymes cleaved directly eIF-4 gamma as part of the purified eIF-4 complex. Addition of HRV2 2A proteinase to HeLa cell cytoplasmic translation extracts resulted in eIF-4 gamma cleavage and drastically reduced the translation of capped mRNA; addition of purified eIF-4 restored translation to the initial level. However, translation of a reporter gene driven by the 5'-untranslated region of human rhinovirus 2 was translated 2-3-fold more efficiently in the presence of HRV2 2A proteinase.

Transcription vectors that facilitate the identification and mapping of RNA splice sites in genomic DNA

Two transcription vectors were constructed that can identify the splice sites at exon-intron boundaries of inserted DNA fragments possessing the complementary splice site. One vector contains the 5' splice donor site and flanking exon-intron sequences from the 3' end of the adenovirus first late leader. The other vector contains the 3' splice acceptor site and the branch acceptor site, plus the flanking exon-intron sequences from the 5' end of the adenovirus second late leader. Both vectors contain a multiple cloning site for insertion of DNA fragments. DNA fragments supplying the complementary splice site, including the adjacent exon and intron sequences, were inserted into the vectors. The vectors were used as templates for the synthesis of chimeric RNA transcripts that were spliced in in vitro splicing extracts. Chimeric transcripts from the vectors containing complementary splice site boundary regions from the human growth hormone gene were accurately spliced in vitro. A splice site from a human growth hormone intron that is not normally spliced in vitro was spliced when paired with an adenovirus splice site. These vectors can be used to identify splice sites and to determine the lengths of exons and their attached introns within a DNA fragment of unknown coding content.

Demonstration of a new antinuclear antibody (anti-RA33) that is highly specific for rheumatoid arthritis

Using immunoblot analysis with soluble nuclear extracts from HeLa cells, we identified autoantibodies to an antigen with a molecular weight of approximately 33,000 in 36% of 95 sera from rheumatoid arthritis patients, but in only 1 of 170 controls. The antigen, termed RA33, was resistant to DNase and RNase digestion but sensitive to proteinase K treatment. There was no discernible relation to other autoantibodies. Thus, this newly described autoantibody appears to be highly specific for rheumatoid arthritis.

Unusual branch point selection involved in splicing of the alternatively processed Calcitonin/CGRP-I pre-mRNA

To study splice site selection in alternative RNA processing we used the human Calcitonin/CGRP-I (CALC-I) gene. Expression of the CALC-I gene in thyroid C-cells results predominantly in calcitonin (CT) mRNA (containing exons 1 to 4) whereas CGRP-I mRNA (containing exons 1,2,3,5 and 6) is the exclusive product in particular nerve cells. We previously reported that a model precursor RNA containing the exon 3 to exon 5 region is predominantly processed into CGRP-I mRNA in vitro using nuclear extracts of three different cell types. To study CT specific processing in Hela cell nuclear extracts we have used precursor RNAs corresponding to the exon 3 to exon 4 region containing only CT specific processing signals. The results revealed the usage of a uridine residue 23 nucleotides upstream of the 3' splice site as the major site of lariat formation in CT specific splicing. The implications of this finding for the alternative, tissue specific processing of the CALC-I pre-mRNA and for branch point selection in general are discussed.

Unusual branch point selection in processing of human growth hormone pre-mRNA

Intron A of the human growth hormone gene does not contain an A residue within 56 nucleotides preceding the 3' splice site. The analysis of the excised intron lariat revealed a C residue 28 nucleotides upstream from the 3' splice site as the major branch acceptor nucleotide. Two additional minor branched nucleotides were identified as U residues at positions -22 and -36. An adenosine substitution at position -22 results in lariat formation solely to this nucleotide. Therefore, C and U residues can function efficiently as natural branch acceptors, but an A residue is preferred if available in the proper region. In addition, the data strongly reinforce the importance of the distance constraint for lariat formation. To explain selection of the branch acceptor nucleotide, potential base-pairing interactions of branch point sequences with the U2 RNA are discussed.

Unusual branch point selection in processing of human growth hormone pre-mRNA

Intron A of the human growth hormone gene does not contain an A residue within 56 nucleotides preceding the 3' splice site. The analysis of the excised intron lariat revealed a C residue 28 nucleotides upstream from the 3' splice site as the major branch acceptor nucleotide. Two additional minor branched nucleotides were identified as U residues at positions -22 and -36. An adenosine substitution at position -22 results in lariat formation solely to this nucleotide. Therefore, C and U residues can function efficiently as natural branch acceptors, but an A residue is preferred if available in the proper region. In addition, the data strongly reinforce the importance of the distance constraint for lariat formation. To explain selection of the branch acceptor nucleotide, potential base-pairing interactions of branch point sequences with the U2 RNA are discussed.

In vitro splicing of adenovirus E1A transcripts: characterization of novel reactions and of multiple branch points abnormally far from the 3' splice site

During the analysis of the in vitro alternative splicing of the natural E1A transcript of adenovirus, other minor reactions were detected (Schmitt et al., 1987, Cell 50, 31-39). We report here their characterization. The first reaction concerns the excision of a 216 nucleotide intron delineated by the 9S 5' splice site and a 3' splice site 216 nucleotides downstream. It can occur on the premRNA transcript and the 13S and 12S mRNA species. Strikingly, the reaction uses one of 3 branch points located 51, 55 or 59 residues upstream of the 3' splice site, a distance which is unusually long since all the branch points mapped up to now are located between 18-37 nucleotides of the 3' splice site. The dramatic accumulation of the corresponding lariat intermediates, likely related to this long spacing indicates that the second splicing step is relatively unefficient. The second kind of reaction analysed is a cryptic splicing which uses a 3' splice site generated by the junction of the 13S mRNA exons, and leads to the formation of psi 12S and psi 9S mRNAs. In vitro, this reaction occurs only from a 13S mRNA transcript, and not from the 13S mRNA newly formed in the splicing assay, consistent with what has been observed in vivo. Thus, both the well known alternative and the minor reactions occurring in vivo from E1A premRNA and mRNAs are detected in vitro, implying that most of the alternative splicing machinery is reconstituted in the in vitro system.