We analyzed the in vitro splicing pathways of three multi-intervening-sequence (IVS) pre-mRNAs: human beta-globin, which contains two IVSs (K. M. Lang, V. L. van Santen, and R. A. Spritz, EMBO J. 4:1991-1996, 1985); rat alpha-lactalbumin, which contains three IVSs; and murine interleukin-3, which contains four IVSs. We found that there are highly preferred pathways of IVS removal from these multi-IVS pre-mRNAs in vitro. The three IVSs of rat alpha-lactalbumin pre-mRNA were excised sequentially from 5' to 3'; in most molecules, IVS1 was removed first, followed by IVS2 and finally by IVS3. The splicing pathway of interleukin-3 pre-mRNA in vitro was more complex. The four IVSs were excised in a highly preferred temporal order, but the order was not strictly sequential or directional. In most molecules, IVS1 and IVS4 were removed first, either simultaneously or in rapid succession. Subsequently, IVS2 was excised, followed by IVS3. The observed splicing pathways apparently resulted from differences in lag times and maximum excision rates of the different IVSs. We detected no exon skipping during splicing of these transcripts in vitro. These observations have implication for proposed models of splice site selection.
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机译：我们分析了三种多中间序列（IVS）前MRNA的体外剪接途径：含有两种IVSS（KM Lang，VL Van Santen和Ra Spritz，Embo J.4：1991-1996 1985年）;大鼠α-乳白蛋白，其中包含三个IVSS;和鼠白细胞介素-3，其中包含四个IVSS。我们发现在体外，IVs的高度优选的IVS从这些多IVS前MRNA中移除。大鼠α-乳白蛋白酶前mRNA的三个IVS依次切除5'至3';在大多数分子中，首先除去IVS1，然后由IVS2除去，最后通过IVS3。体外白细胞介素-3前mRNA的剪接途径更复杂。四个IVS以高度优选的时间顺序切除，但订单并不严格顺序或定向。在大多数分子中，首先，同时或快速地除去IVS1和IVS4。随后，切除IVS2，然后是IVS3。观察到的剪接途径显然是由于滞后时间的差异和不同IVS的最大切除率。我们在体外剪接这些转录物期间没有检测到外显子跳跃。这些观察结果对所提出的剪接部位选择模型有所意义。
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