Conditionally lethal mutant alleles of theFUSCA3(FUS3) gene ofArabidopsis thalianaare specifically defective in the gene expression program responsible for seed maturation.FUS3was isolated by map-based cloning and expression of theFUS3cDNA 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 byABI3(Arabidopsis) andVP1(maize).FUS3transcription was detected mainly in siliques and was found to be developmentally regulated during embryogenesis. Transcripts of abnormal sizes were observed infus3mutants due to aberrant splicing caused by point mutations at intron termini. Sequence analysis of mutant and wild-typeFUS3alleles, as well as sequencing offus3cDNAs, revealed small in-frame 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.