Characterization of an alpha-amylase multigene cluster in rice

Rice genomic clones containing eight different alpha-amylase genes have been previously classified into five groups based on DNA hybridization studies and restriction site mapping. This report describes the clustering of three Group 3 genes (RAmy3A, RAmy3B and RAmy3C) within 28 kb of genomic DNA. The genes are separated from each other by about 5 kb and transcribed in the same direction. At the protein level, RAmy3B and RAmy3C are 95% homologous while each is 78% homologous to RAmy3A. All three genes have relatively small introns in the first and third positions. RAmy3A; however, has an additional 409 bp intron in the second intron insertion site. Nucleotide sequence comparisons of the coding and 3' flanking regions suggest that clustering of the RAmy3 genes occurred by gene duplication resulting from unequal crossing-over at repetitive sequences. A comparison of the 5' flanking regions revealed several sequences that may be involved in transcription. Expression of RAmy3B/C first appears in the germinating seed after two days and at a higher level after four days. Quantitative primer extension analysis indicates that RAmy3B and RAmy3C contribute 25% and 75%, respectively, of the transcripts from this cluster at four days of germination. No primer extension band specific to RAmy3A transcripts could be detected at this time point. However, RAmy3A PCR products could be amplified from RNA isolated from embryo-derived callus tissue.