Kronert W A, Edwards K A, Roche E S, Wells L, Bernstein S I
Biology Department, San Diego State University, CA 92182.
EMBO J. 1991 Sep;10(9):2479-88. doi: 10.1002/j.1460-2075.1991.tb07787.x.
We show that the molecular lesions in two homozygousviable mutants of the Drosophila muscle myosin heavy chain gene affect an alternative exon (exon 9a) which encodes a portion of the myosin head that is highly conserved among both cytoplasmic and muscle myosins of all organisms. In situ hybridization and Northern blotting analysis in wild-type organisms indicates that exon 9a is used in indirect flight muscles whereas both exons 9a and 9b are utilized in jump muscles. Alternative exons 9b and 9c are used in other larval and adult muscles. One of the mutations in exon 9a is a nonsense allele that greatly reduces myosin RNA stability. It prevents thick filament accumulation in indirect flight muscles and severely reduces the number of thick filaments in a subset of cells of the jump muscles. The second mutation affects the 5' splice site of exon 9a. This results in production of an aberrantly spliced transcript in indirect flight muscles, which prevents thick filament accumulation. Jump muscles of this mutant substitute exon 9b for exon 9a and consequently have normal levels of thick filaments in this muscle type. This isoform substitution does not obviously affect the ultrastructure or function of the jump muscle. Analysis of this mutant illustrates that indirect flight muscles and jump muscles utilize different mechanisms for alternative RNA splicing.
我们发现，果蝇肌肉肌球蛋白重链基因的两个纯合可存活突变体中的分子损伤影响一个可变外显子（外显子9a），该外显子编码肌球蛋白头部的一部分，在所有生物体的细胞质肌球蛋白和肌肉肌球蛋白中都高度保守。在野生型生物体中进行的原位杂交和Northern印迹分析表明，外显子9a在间接飞行肌中使用，而外显子9a和9b在跳跃肌中都被利用。可变外显子9b和9c在其他幼虫和成虫肌肉中使用。外显子9a中的一个突变是一个无义等位基因，它大大降低了肌球蛋白RNA的稳定性。它阻止了间接飞行肌中粗肌丝的积累，并严重减少了跳跃肌一部分细胞中的粗肌丝数量。第二个突变影响外显子9a的5'剪接位点。这导致在间接飞行肌中产生异常剪接的转录本，从而阻止粗肌丝的积累。该突变体的跳跃肌用外显子9b替代外显子9a，因此这种肌肉类型中的粗肌丝水平正常。这种异构体替代并没有明显影响跳跃肌的超微结构或功能。对该突变体的分析表明，间接飞行肌和跳跃肌利用不同的机制进行可变RNA剪接。
