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Conditionally lethal mutant alleles of the FUSCA3 (FUS3) gene of Arabidopsis thaliana are specifically defective in the gene expression program responsible for seed maturation. FUS3 was isolated by map-based cloning and expression of the FUS3 cDNA resulted in complementation of the Fus3- phenotype. In the predicted FUS3 gene product, a continuous stretch of more than 100 amino acids shows significant sequence similarity to the B3 domains of the polypeptides encoded by ABI3 (Arabidopsis) and VP1 (maize). FUS3 transcription was detected mainly in siliques and was found to be developmentally regulated during embryogenesis. Transcripts of abnormal sizes were observed in fus3 mutants due to aberrant splicing caused by point mutations at intron termini. Sequence analysis of mutant and wild-type FUS3 alleles, as well as sequencing of fus3 cDNAs, revealed small inframe deletions at two different sites of the coding region. While a deletion between B3 and the C-terminus of the predicted polypeptide was found in conjunction with normal FUS3 function, another deletion located within the conserved B3 domain (as well as truncations therein) were associated with the Fus3- phenotype. It is apparent, therefore, that an intact B3 domain is essential for the regulation of seed maturation by FUS3.

The Arabidopsis protein COP1, encoded by the constitutive photomorphogenic locus 1, is an essential regulatory molecule that plays a role in the repression of photomorphogenic development in darkness and in the ability of light-grown plants to respond to photoperiod, end-of-day far-red treatment, and ratio of red/far-red light. The COP1 protein contains three recognizable structural domains: starting from the N terminus, they are the zinc binding motif, the putative coiled-coil region, and the domain with multiple WD-40 repeats homologous to the beta subunit of trimeric G-proteins (G beta). To understand the functional implications of these structural motifs, 17 recessive mutations of the COP1 gene have been isolated based on their constitutive photomorphogenic seedling development in darkness. These mutations define three phenotypic classes: weak, strong, and lethal. The mutations that fall into the lethal class are possible null mutations of COP1. Molecular analysis of the nine mutant alleles that accumulated mutated forms of COP1 protein revealed that disruption of the G beta-protein homology domain or removal of the very C-terminal 56 amino acids are both deleterious to COP1 function. In-frame deletions or insertions of short amino acid stretches between the putative coiled-coil and G beta-protein homology domains strongly compromised COP1 function. However, a mutation resulting in a COP1 protein with only the N-terminal 282 amino acids, including both the zinc binding and the coiled-coil domains, produced a weak phenotypic defect. These results indicated that the N-terminal half of COP1 alone retains some activity and a disrupted C-terminal domain masks this remaining activity.