Analysis of a Novel Mechanism of Genetic Reversion in Arabidopsis

遗传物质脱氧核糖核酸（DNA）的稳定性和完整性依赖于无数的细胞过程。由于大多数生物体在成熟后继续经历细胞分裂，因此基因组的复制和修复对于生物体的整个生命周期的正常生长和发育仍然至关重要。为了确保基因组的完整性，存在许多普遍且相对较好理解的DNA修复系统。近年来，核糖核酸（RNA）的重要作用已被证明远远超过信使RNA、转移RNA和核糖体RNA。RNA已被证明在各种RNA酶中起催化作用，最近已被证明通过靶向特定信使RNA进行降解来调节基因表达。该研究将描述一种新的遗传逆转机制，它可能代表一种模板定向DNA修饰的情况，其中指导修饰的模板是RNA。具体来说，该项目旨在了解模式植物拟南芥（Arabidopsis thaliana）中一系列突变体中遗传不稳定性的本质，这些突变体在HOTHEAD基因中存在遗传损伤。尽管HOTHEAD基因产物本身与一类参与氰化物降解的酶只有表面的同源性，并且不具有任何典型的dna相关蛋白的基序，但该基因似乎易受一种新型遗传逆转的影响。热头等位基因的单点突变以高频率恢复到野生型核苷酸序列。此外，来自杂合子热头亲本植物的基因典型野生型植物也表现出类似的不稳定性，但在这种情况下，涉及到突变等位基因的改变，这似乎是一种等位基因特异性的方式。在HOTHEAD基因中看到的遗传物质的不稳定性可能代表了一种一般机制，该机制允许生物体在染色体中存在的每个基因的两个等位基因之外保持额外的DNA序列变异。

The stability and integrity of the genetic material, deoxyribonucleic acid (DNA), is dependent upon a myriad of cellular processes. Since most organisms continue to experience cell division after achieving maturity, replication and repair of the genome remain essential for normal growth and development throughout the life span of that organism. A host of universal and relatively well-understood DNA repair systems exist to ensure genomic integrity. In recent years, ribonucleic acid (RNA) has been demonstrated to play important roles far beyond those of messenger RNA, transfer RNA and ribosomal RNA. RNA has been shown to play a catalytic role in various RNAse enzymes and most recently has been shown to play a role in regulating gene expression by targeting specific messenger RNAs for degradation. The research to be carried out will characterize a novel mechanism of genetic reversion that may represent a case of template-directed DNA modification where the template directing the modification is RNA. Specifically, this project aims to understand the nature of the genetic instability seen in a collection of mutants in the model plant, Arabidopsis thaliana, that have genetic lesions in the HOTHEAD gene. Although the HOTHEAD gene product itself shares only superficial homology with a class of enzymes involved in degradation of cyanogenic compounds and does not feature any motifs typical of DNA-associated proteins, this gene appears to be susceptible to a novel type of genetic reversion. Single point mutations in hothead alleles revert to the wild-type nucleotide sequence at a high frequency. Furthermore, genotypically wild-type plants derived from heterozygous hothead parent plants show a similar instability, but in this case involving alteration back to the mutant allele in what appears to be an allele-specific manner. The instability of the genetic material seen in the HOTHEAD gene may represent a general mechanism that allows the organism to maintain additional DNA sequence variation beyond the two alleles of each gene present in the chromosomes.