Saidapet C R, Munro H N, Valgeirsdóttir K, Sarkar S
Proc Natl Acad Sci U S A. 1982 May;79(10):3087-91. doi: 10.1073/pnas.79.10.3087.
The emergence of abundant-class mRNAs specific for contractile muscle proteins and their distribution between polysomal and free mRNP fractions were studied in skeletal muscle excised from chicken embryos during the transition from myoblasts (day 9) to myotubes (day 18). Muscle-specific cDNA was selectively prepared by hybridizing cDNA to template RNA (polysomal poly(A)+ mRNA) from day-14 embryos followed by isolation of the abundant class, which represents approximately 20% of total mRNA. The specificity of the cDNA probe for this class was confirmed by the differential degree of hybridization to cytoplasmic RNA from cultured myotube and myoblast cells and by its inability to hybridize with mRNA from nonmuscle cells such as liver. Except for muscle from day-9 embryos, the concentrations of the abundant-class muscle-specific mRNAs were higher in polysomes than in free mRNP fractions. Furthermore, the levels of these mRNAs in polysomes increased 12-fold from day 9 (myoblast) to day 14 (intermediate) with a further 3.6-fold increase from day 14 to day 18 (myotube). In contrast to this 45-fold net increase in the polysomal level of these mRNAs from day 9 to day 18, the levels in the free mRNP fraction showed only a 3-fold decrease during this period. Because the amount of mRNA lost from the mRNP fraction is much less than the net increase in the polysome fraction, mRNP does not serve as a reservoir of untranslated muscle-specific mRNA for transfer to polysomes. Consequently, the emergence of muscle-specific polysomal mRNA for contractile proteins during myogenesis in ovo appears to be regulated primarily by transcriptional control.
在鸡胚骨骼肌从成肌细胞（第9天）向肌管（第18天）转变过程中，研究了收缩性肌肉蛋白特异性的丰富类mRNA的出现及其在多核糖体和游离mRNA核糖核蛋白颗粒组分之间的分布。通过将cDNA与来自第14天胚胎的模板RNA（多核糖体聚腺苷酸加mRNA）杂交，然后分离出丰富类，选择性地制备了肌肉特异性cDNA，该丰富类约占总mRNA的20%。通过与培养的肌管和成肌细胞的细胞质RNA杂交的差异程度以及其不能与非肌肉细胞（如肝脏）的mRNA杂交，证实了该cDNA探针针对此类的特异性。除了第9天胚胎的肌肉外，多核糖体中丰富类肌肉特异性mRNA的浓度高于游离mRNA核糖核蛋白颗粒组分中的浓度。此外，这些mRNA在多核糖体中的水平从第9天（成肌细胞）到第14天（中间阶段）增加了3.6倍，从第14天到第18天（肌管）又进一步增加了3.6倍。与这些mRNA从第9天到第18天在多核糖体水平上45倍的净增加形成对比的是，游离mRNA核糖核蛋白颗粒组分中的水平在此期间仅下降了3倍。由于从mRNA核糖核蛋白颗粒组分中丢失的mRNA量远小于多核糖体组分中的净增加量，mRNA核糖核蛋白颗粒并不作为未翻译的肌肉特异性mRNA转移到多核糖体的储存库。因此，在胚胎发育过程中，收缩蛋白的肌肉特异性多核糖体mRNA的出现似乎主要受转录控制。
