Sehgal D, Mittal V, Ramachandran S, Dhar S K, Bhattacharya A, Bhattacharya S
School of Life Sciences, Jawaharlal Nehru University, New Delhi, Indiana.
Mol Biochem Parasitol. 1994 Oct;67(2):205-14. doi: 10.1016/0166-6851(94)00129-4.
We have sequenced the extrachromsomal ribosomal DNA (rDNA) circle of the human protozoan parasite Entamoeba histolytica HM-1:IMSS and present here the complete sequence organisation of the 24.5-kb molecule. Each circle contains two 5.9-kb rDNA transcription units organised as inverted repeats. The regions downstream (3543 bp) and upstream (9216 bp) of the rDNAs contain various families of short tandem repeats. Some of the upstream repeats share extensive sequence homology with the downstream repeats. In addition to the rDNAs themselves, the rDNA circle appears to code for only one other transcript which is 0.7 kb in size as seen in Northern blots. From DNA sequence analysis, no open reading frame could be assigned to the transcript. Extrachromosomal rDNA circles also exist in other E. histolytica strains. Restriction enzyme maps of rDNA circles were constructed from E. histolytica strains 200:NIH, HK-9 and Rahman; and Entamoeba moshkovskii strain Laredo. Striking differences were observed in the organisation of some of them, e.g. the HK-9, Rahman and Laredo circles contained only one rDNA unit and lacked the 0.7-kb transcript sequence. The short repeat sequences upstream and downstream of rDNAs were present in HK-9 and Rahman but absent in Laredo. Circles with one rDNA unit may be derived from those with two units by homologous recombination at direct repeat sequences located upstream and downstream of the two rDNAs.
我们对人类原生动物寄生虫溶组织内阿米巴HM-1:IMSS的染色体外核糖体DNA（rDNA）环进行了测序，并在此展示了这个24.5kb分子的完整序列结构。每个环包含两个以反向重复形式排列的5.9kb rDNA转录单元。rDNA下游（3543bp）和上游（9216bp）区域包含各种短串联重复序列家族。一些上游重复序列与下游重复序列具有广泛的序列同源性。除了rDNA本身，rDNA环似乎只编码另一种转录本，如在Northern印迹中所见，其大小为0.7kb。通过DNA序列分析，无法为该转录本指定开放阅读框。染色体外rDNA环也存在于其他溶组织内阿米巴菌株中。从溶组织内阿米巴菌株200:NIH、HK-9和拉赫曼，以及莫氏内阿米巴菌株拉雷多构建了rDNA环的限制性酶切图谱。在它们的结构中观察到了显著差异，例如HK-9、拉赫曼和拉雷多的环只包含一个rDNA单元，并且缺乏0.7kb的转录本序列。rDNA上游和下游的短重复序列在HK-9和拉赫曼中存在，但在拉雷多中不存在。具有一个rDNA单元的环可能是通过位于两个rDNA上游和下游的直接重复序列处的同源重组，从具有两个单元的环衍生而来的。
