A nuclear AmyB gene from sweet potato encoding beta-amylase (beta Amy) that is abundant in tuberous roots and inducible in other organs by an exogenous supply of sucrose or polygalacturonic acid, was isolated and characterized. Genomic Southern blot hybridization, restriction maps of independently isolated phage lambda genomic clones, and the nucleotide sequence of AmyB compared with that of the cDNA, all suggested that beta Amy of sweet potato is encoded by a gene that is present in a single copy per haploid genome. In the sequence of AmyB, the sequence that is identical to that of the cDNA was split into seven exons by six introns, and the transcription of this gene starts from multiple sites 26 to 30 bp downstream from a potential TATA-box sequence, 5'-TATATAA. In the 5'-upstream region of AmyB, there are sequences homologous to those conserved in the 5'-upstream regions of genes encoding sporamin, which are regulated similarly to AmyB. The 5'-upstream region of AmyB also contains sequences to which several previously known plant nuclear factors bind.