Analysis of a Novel Mechanism of Genetic Reversion in Arabidopsis

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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.