陆地棉线粒体nad6基因mRNA无常规终止密码子

Upland Cotton Mitochondrial nad6 mRNAs Lack Standard Stop Codon

植物线粒体nad6基因编码NADH(还原型辅酶Ⅰ)脱氢酶第6亚基,前期研究发现,该基因可能与棉花细胞质雄性不育相关,但该基因的转录情况尚不清楚.本研究利用PCR测序、Southern印迹方法发现,陆地棉线粒体基因组(mtDNA)中nad6基因长621 bp,且为单拷贝.RT-PCR及环化RT-PCR分析发现,其mRNA在终止密码子前-14或-15 nt处提前终止;虽然该基因mRNA编码区存在12处RNA编辑(C-U)位点,但并未产生新的替代的终止密码子;基因mRNAs尾端poly(A)处含有0、1、2或4个"A",也并未与前方相邻碱基凑成新的终止密码子,即:该基因mRNA无常规终止密码子(UGA,UAG,UAA).本研究结果提示,棉花线粒体nad6基因mRNA很可能有其它的终止密码子.针对这种情况对植物线粒体如何翻译无常规终止密码子的mRNA进行了讨论.

The plant mitochondrial nad6 gene codes for subuint 6 of the NADH-ubiquinone oxidoreductase. Previous studies showed that it might be associated with cotton cytoplasmic male sterility.But the transcription profile of this gene remained unclear. The genomic DNA sequence of nad6 gene in the mitochondrial genome of Upland cotton( Gossypium hirsutum L.) was 621 bp,and existed as a single copy. To get further knowledge of the expression of nad6 gene,we used Northern blotting,RT-PCR,and circular RT-PCR( cRT-PCR) methods to analyze the transcription of this gene. The results showed that its mRNAs were terminated and processed at-14 nt or-15 nt upstream of the stop codon. Comparison of its cDNAs and genomic sequences revealed 12 RNA editing sites in the coding region,all of them corresponding to C-U conversions. And the poly( A) region at the termini of mRNAs had 0,1,2,or 4'A's. But there was no evidence that an alternative stop codon was created,neither by RNA editing nor by polyadenylation. This meant that cotton mitochondrial nad6 gene lacked standard stop codon( UGA,UAG,UAA). The results indicated that another alternative stop codon might exist in the mRNAs of nad6 gene. Discussion was made on how the mRNAs lacking standard stop codon were translated.