CHARACTERIZATION OF ACTIVE TRANSCRIPTION UNITS IN BALBIANI RINGS OF CHIRONOMUS TENTANS
CHARACTERIZATION OF ACTIVE TRANSCRIPTION UNITS IN BALBIANI RINGS OF CHIRONOMUS TENTANS
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DOI：
10.1016/0092-8674(79)90324-6
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发表时间：
1979-01-01
期刊：
影响因子：
64.5
通讯作者：
DANEHOLT, B
中科院分区：
文献类型：
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作者：
LAMB, MM;DANEHOLT, B
利用Miller扩散技术和常规电镜原位观察，发现了4号染色体上特定的活性转录单位。这些单位可能位于4号染色体上最明显的2个突起，即巴尔比亚尼环1 （Balbiani ring 1）和巴尔比亚尼环2 （Balbiani ring 2）。这些Balbiani环中的转录单位产生75S RNA分子，构成主要细胞产物唾液多肽的推定mRNA物种。单生活性转录单位平均长度为7.7 .mu。M是观察最频繁的。每个单元的外侧核糖核蛋白（RNP）纤维形成一个单一的长度梯度。每单位纤维数为123根。24)或每亩大约生长16根RNP纤维。染色体纤维的M。每单位RNP纤维数量的显著变化表明转录可以在单个基因水平上进行调节。调制可能是通过起始事件和/或通过早期的预终止步骤实现的，但不能排除伸长率的变化。开始穿越整个基因的聚合酶的数量为。6/min和转录单位，在18度下计算RNA链延伸率为31个核苷酸/s。C.在Miller菌株中，研究了活性75S RNA单元内及其附近染色体纤维的特性。失活的染色体纤维呈均匀的串珠状，而有活性的染色体纤维呈稀疏的不规则串珠状。染色体纤维在有活性的75S RNA单元比在无活性区域更延伸（DNA包装比分别为1.6和1.9）。通过比较活性75S RNA基因与其他系统中活性基因的特性，我们推测活性单元中珠粒的丢失和纤维的延伸可能与转录活性水平直接相关。最后，获得了0.18 .亩的光滑无珠段。在RNP纤维梯度之前发现长度为m。这个片段可能在转录调控过程中起作用。基于与扩散制剂中的活性转录单元的比较，可以使用常规EM在切片材料中识别出Balbiani环中的活性单元。在BR 1和BR 2中，活性单元表现为一个环，由纤维轴组成，RNP颗粒通过秸秆附着在环轴上。因此，在转录过程中，生长的RNP纤维似乎被组织成颗粒结构，br1和br2的最终产物是颗粒，500 . ang。直径为1毫米，每个含有一个75S RNA分子。
Specific active transcription units on chromosome IV in the salivary glands of C. tentans were visualized by the Miller spreading technique and in situ by conventional EM. These units are likely to be located in the 2 most conspicuous puffs on chromosome IV, Balbiani ring 1 (BR 1) and Balbiani ring 2 (BR 2). The transcription units in these Balbiani rings generate 75S RNA molecules constituting putative mRNA species for the predominant cellular product, the salivary polypeptides. Solitary active transcription units with a mean length of 7.7 .mu.m were observed most frequently. The lateral ribonucleoprotein (RNP) fibers of each unit formed a single length gradient. The number of fibers per unit was 123 (.+-. 24) or about 16 growing RNP fibers per .mu.m of chromosome fiber. The considerable variation in the number of RNP fibers per unit suggests that transcription can be modulated at the level of the individual gene. The modulation is probably achieved via the initiation event and/or via an early pretermination step, but a change in the elongation rate could not be excluded. The number of polymerases starting to traverse the whole gene was .apprx. 6/min and transcription unit and the rate of RNA chain elongation was calculated to be 31 nucleotides/s at 18.degree. C. The properties of the chromosome fiber within the active 75S RNA units and also within their vicinity were studied in the Miller spreads. The inactive chromosome fiber exhibited a uniform beaded conformation, while the active fiber was sparsely and irregularly beaded. The chromosome fiber was more extended in the active 75S RNA unit than in inactive regions (DNA packing ratios of 1.6 and 1.9, respectively). By comparing the properties of the active 75S RNA gene with those of active genes in other systems, it was inferred that the loss of beads and the extension of the fiber in the active unit is probably directly related to the level of transcriptive activity. Finally, a smooth nonbeaded segment of 0.18 .mu.m in length was found to precede the RNP fiber gradient. This segment may have a role in the process of transcriptional regulation. Based on comparison with the active transcription units in spread preparations, it was possible to identify active units in the Balbiani rings in sectioned material using conventional EM. In BR 1 and BR 2 an active unit appeared as a loop, consisting of a fiber axis and having RNP granules attached to the loop axis by stalks. The growing RNP fibers therefore seem to be organized into granular structures during the transcription process and the final products in BR 1 and BR 2 are granules, 500 .ANG. in diameter, each containing a 75S RNA molecule.