The ABI3 gene in Arabidopsis was isolated using positional cloning. Researchers constructed a detailed restriction fragment length polymorphism (RFLP) map of the abi3 region and identified an RFLP marker closely linked to the ABI3 locus. By analyzing overlapping cosmid clones containing this marker, the ABI3 locus was localized within a 35-kb region. An 11-kb subfragment was shown to complement the mutant phenotype in transgenic plants, narrowing the locus's position. A candidate ABI3 gene was identified within this fragment, expressed in developing fruits. The primary structure of the encoded protein was determined from a cDNA clone. In the most severe abi3-4 allele, a point mutation introduced a premature stop codon, reducing the protein size by 40%. The ABI3 protein showed high similarity to the maize viviparous-1 protein. The study highlights the feasibility of isolating Arabidopsis genes through positional cloning. This method is particularly useful for genes identified only by mutant phenotypes. The ABI3 gene was found to be involved in abscisic acid (ABA) signaling, with the protein likely participating in ABA perception and transduction pathways. Sequence analysis revealed that ABI3 shares similarities with the maize VP1 protein, suggesting a conserved function in ABA signaling. The ABI3 protein contains a putative nuclear targeting sequence and several domains with features of transcriptional activation domains, indicating its role in ABA-regulated gene expression during seed development. The study also demonstrated that the ABI3 gene is expressed in developing siliques and that the most severe abi3-4 mutant allele contains a mutation leading to a truncated protein. The identification of the ABI3 gene provides valuable insights into ABA signaling pathways and their regulation in plants. The research underscores the importance of positional cloning in gene isolation and highlights the potential for further investigation into the functional properties and physiological roles of the ABI3 protein.The ABI3 gene in Arabidopsis was isolated using positional cloning. Researchers constructed a detailed restriction fragment length polymorphism (RFLP) map of the abi3 region and identified an RFLP marker closely linked to the ABI3 locus. By analyzing overlapping cosmid clones containing this marker, the ABI3 locus was localized within a 35-kb region. An 11-kb subfragment was shown to complement the mutant phenotype in transgenic plants, narrowing the locus's position. A candidate ABI3 gene was identified within this fragment, expressed in developing fruits. The primary structure of the encoded protein was determined from a cDNA clone. In the most severe abi3-4 allele, a point mutation introduced a premature stop codon, reducing the protein size by 40%. The ABI3 protein showed high similarity to the maize viviparous-1 protein. The study highlights the feasibility of isolating Arabidopsis genes through positional cloning. This method is particularly useful for genes identified only by mutant phenotypes. The ABI3 gene was found to be involved in abscisic acid (ABA) signaling, with the protein likely participating in ABA perception and transduction pathways. Sequence analysis revealed that ABI3 shares similarities with the maize VP1 protein, suggesting a conserved function in ABA signaling. The ABI3 protein contains a putative nuclear targeting sequence and several domains with features of transcriptional activation domains, indicating its role in ABA-regulated gene expression during seed development. The study also demonstrated that the ABI3 gene is expressed in developing siliques and that the most severe abi3-4 mutant allele contains a mutation leading to a truncated protein. The identification of the ABI3 gene provides valuable insights into ABA signaling pathways and their regulation in plants. The research underscores the importance of positional cloning in gene isolation and highlights the potential for further investigation into the functional properties and physiological roles of the ABI3 protein.