Katoh M, Hirono M, Takemasa T, Kimura M, Watanabe Y
Institute of Biological Sciences, University of Tsukuba, Ibaraki, Japan.
Nucleic Acids Res. 1993 May 25;21(10):2409-14. doi: 10.1093/nar/21.10.2409.
Tetrahymena thermophila possesses a transcriptionally inactive micronucleus and an active macronucleus. Both nuclei are developed from micronucleus-derived germ nuclei during conjugation. Extensive DNA rearrangement and transcriptional activation are known to be involved in macronuclear development, but little has been known about these processes in a particular functional gene. Therefore the micro- and macronuclear genomic DNAs for calmodulin gene were analyzed. A 1,384 bp micronucleus-specific sequence located about 3.5 kb upstream of calmodulin gene has been found, suggesting DNA rearrangement during macronuclear development. The micronucleus-specific sequence had 85% A + T, no extensive ORF, ATTAs at both ends, and two palindromic structures just outside of both ends. Interestingly, the micronucleus-specific sequence included a T-rich tract, T16CT5, in the middle, and a nearly complementary A-rich tract, A5TA10GA5, existed 7 bp upstream from the initiation codon. In addition, there was a 20 bp repetitive sequence TAAT(TAAC)4 about 100 bp upstream of the micronucleus-specific sequence and also in the promoter region of calmodulin gene. Although the functional significance of the micronucleus-specific sequence remains unclear, T16CT5 and TAAT(TAAC)4 elements might exert an influence on transcription of the calmodulin gene. Stringent Southern hybridization revealed that this micronucleus-specific sequence or very similar sequence(s) were abundant in the Tetrahymena micronuclear genome.
嗜热四膜虫拥有一个转录不活跃的微核和一个活跃的大核。在接合过程中，两个核均由源自微核的生殖核发育而来。已知广泛的DNA重排和转录激活参与大核发育，但对于特定功能基因的这些过程了解甚少。因此，对钙调蛋白基因的微核和大核基因组DNA进行了分析。已发现位于钙调蛋白基因上游约3.5 kb处的一个1384 bp的微核特异性序列，这表明在大核发育过程中发生了DNA重排。该微核特异性序列的A + T含量为85%，没有广泛的开放阅读框，两端有ATTAs，两端外侧各有两个回文结构。有趣的是，微核特异性序列中间包含一个富含T的区域T16CT5，在起始密码子上游7 bp处存在一个几乎互补的富含A的区域A5TA10GA5。此外，在微核特异性序列上游约100 bp处以及钙调蛋白基因的启动子区域还存在一个20 bp的重复序列TAAT(TAAC)4。尽管微核特异性序列的功能意义尚不清楚，但T16CT5和TAAT(TAAC)4元件可能会对钙调蛋白基因的转录产生影响。严格的Southern杂交显示，这种微核特异性序列或非常相似的序列在嗜热四膜虫微核基因组中很丰富。
