Mochizuki Kazufumi, Gorovsky Martin A
Department of Biology, University of Rochester, 425 Hutchison Hall, Rochester, NY 14627, USA.
Eukaryot Cell. 2004 Oct;3(5):1233-40. doi: 10.1128/EC.3.5.1233-1240.2004.
The germ line micronucleus in Tetrahymena thermophila is transcriptionally silent in vegetatively growing cells. However, micronuclear transcription has been observed in the early ("crescent") stages of the sexual process, conjugation. This transcription is proposed to play a central role in identifying sites for subsequent genome rearrangements that accompany development of the somatic macronucleus from the micronucleus. RPB3 (cnjC), a gene encoding a protein homologous to the third largest subunit of RNA polymerase II (RNAP II), was previously reported to be expressed specifically during conjugation, suggesting a role in micronucleus-specific transcription. Rpb3p localized in the micronucleus only during the meiotic prophase, when micronuclear transcription occurs, and its intranuclear distribution is strikingly similar to that for previously described sites of micronuclear RNA synthesis. By contrast, Rpc5p, the homologous subunit shared by RNAPs I and III, was not detectable in the micronucleus at any stage of the life cycle. However, Rpb3p is not specific to the transcribing micronucleus. Like Rpc5p, it also localizes to macronuclei in all stages of the life cycle. Rpb3p is encoded by a unique, essential gene in Tetrahymena. Thus, RNAP II is associated with both somatic transcription and crescent transcription and probably has an important role in genome rearrangement.
嗜热四膜虫的生殖系微核在营养生长细胞中是转录沉默的。然而，在有性过程（接合）的早期（“新月形”）阶段观察到了微核转录。这种转录被认为在识别随后伴随微核发育成体细胞大核的基因组重排位点方面发挥核心作用。RPB3（cnjC）是一个编码与RNA聚合酶II（RNAP II）第三大亚基同源的蛋白质的基因，先前报道它在接合过程中特异性表达，提示其在微核特异性转录中发挥作用。Rpb3p仅在减数分裂前期微核转录发生时定位于微核，其在核内的分布与先前描述的微核RNA合成位点惊人地相似。相比之下，RNAP I和III共有的同源亚基Rpc5p在生命周期的任何阶段在微核中均未检测到。然而，Rpb3p并非转录微核所特有。与Rpc5p一样，它在生命周期的所有阶段也定位于大核。Rpb3p由嗜热四膜虫中一个独特的必需基因编码。因此，RNAP II与体细胞转录和新月形转录均相关，并且可能在基因组重排中起重要作用。
