Curtis P J
Biochim Biophys Acta. 1980 Sep 22;605(3):347-64. doi: 10.1016/0304-419x(80)90016-5.
Accumulation of haemoglobin in induced MEL cells begins with the activation of transcription of the globin genes. Though much has been learned of cellular events affecting expression of the globin genes, e.g., from noninducible variants of MEL cells and cell fusion between MEL cells and other cell types, there is at present no in vitro system available that would permit more detailed study of the molecular events leading to transcription of the globin genes. Presumably, with the availability of cloned chromosomal genes such systems will soon be found. The products of transcription detected in induced MEL cells are 15 S and 11 S species which are precursor forms of beta- and alpha-globin mRNA, respectively. An unmodified primary transcript has not been detected. The 15 S species possesses a fully methylated 'cap' 1 structure the 5' end and poly(A) at the 3' end. Conceivably, there could be cleaving or splicing events preceding the 'capping' and polyadenylation, but all these reactions must occur extremely rapidly, since with a t 1/2 approx. 2 min a large proportion of the 15 S beta-globin RNA must be newly synthesised. It also contains the two intervening sequences found in the chromosomal genes. Selective processing occurs since from pulse and pulse-chase experiments most if not all of the 15 S beta-globin RNA is processed to mature 10 S beta-globin RNA very rapidly, whereas less than 10% of newly synthesised nuclear RNA (HnRNA) leaves the nucleus, the remainder being hydrolysed in the nucleus with a t 1/2 approx. 20 min. Perhaps rapid processing permits efficient transport to the cytoplasm. Further processing occurs in steps; apparently the large intervening sequence is removed first followed by the small intervening sequence. These steps do not appear to be rate limiting events and these sequences have not been detected separately from the 15 S beta-globin RNA. Such results and the wide divergency of intervening sequences, suggest that the intervening sequences per se play no essential function in the cell, though their presence in the nuclear transcript appears to necessary for processing to the mRNA. The selective processing accounts for more increase of the globin RNA in MEL cells, and further accumulation occurs by virtue of the stability of globin mRNA (t 1/2 approx. 17 h) compared with the bulk of poly(A)-RNA (t 1/2 approx. 3 h). It would appear, however, that specific destabilization of a class of stable mRNA (t 1/2 approx. 35 h) is necessary to allow globin mRNA to asccount for 90% of the mRNA population in reticulocytes.
在诱导的MEL细胞中，血红蛋白的积累始于珠蛋白基因转录的激活。尽管已经了解了许多影响珠蛋白基因表达的细胞事件，例如从MEL细胞的非诱导性变体以及MEL细胞与其他细胞类型之间的细胞融合中所学到的，但目前尚无可用的体外系统能够更详细地研究导致珠蛋白基因转录的分子事件。据推测，随着克隆染色体基因的可得性，这样的系统很快就会被找到。在诱导的MEL细胞中检测到的转录产物是15S和11S物种，它们分别是β-和α-珠蛋白mRNA的前体形式。未检测到未修饰的初级转录本。15S物种在5'端具有完全甲基化的“帽”1结构，在3'端具有多聚腺苷酸。可以想象，在“加帽”和多聚腺苷酸化之前可能存在切割或剪接事件，但所有这些反应必定极其迅速地发生，因为其半衰期约为2分钟，很大一部分15Sβ-珠蛋白RNA必定是新合成的。它还包含在染色体基因中发现的两个间隔序列。由于脉冲和脉冲追踪实验表明，大部分（如果不是全部）15Sβ-珠蛋白RNA会非常迅速地加工成成熟的10Sβ-珠蛋白RNA，而新合成的核RNA（HnRNA）不到10%离开细胞核，其余的在细胞核中以约20分钟的半衰期被水解，所以发生了选择性加工。也许快速加工允许有效地转运到细胞质中。进一步的加工是分步骤进行的；显然，大的间隔序列首先被去除，随后是小的间隔序列。这些步骤似乎不是限速事件，并且这些序列尚未与15Sβ-珠蛋白RNA分开检测到。这些结果以及间隔序列的广泛差异表明，间隔序列本身在细胞中不发挥重要功能，尽管它们在核转录本中的存在对于加工成mRNA似乎是必要的。选择性加工解释了MEL细胞中珠蛋白RNA的更多增加，并且由于珠蛋白mRNA（半衰期约为17小时）与大部分多聚腺苷酸RNA（半衰期约为3小时）相比具有稳定性，所以进一步积累得以发生。然而，似乎一类稳定的mRNA（半衰期约为35小时）的特异性去稳定化对于使珠蛋白mRNA在网织红细胞中占mRNA群体的90%是必要的。
