Three families of retrotransposons of rice (Tos1, Tos2, and Tos3) were isolated by using a method based on the sequence conservation of the primer binding site for reverse transcription. This method should be generally applicable for cloning retrotransposon of other plants. One retrotransposon, Tos3-1, was studied in detail. Tos3-1 is 5.2 kb long, has structures common to retrotransposons, such as long terminal repeats (LTR), a primer binding site complementary to the initiator tRNA, a polypurine tract, and generates target sequence duplications flanking the inserted element. Southern blotting analysis showed that sequences homologous to Tos1, 2 and 3 are found in wild rice species as well as in cultivated rice species, but not in maize and tobacco. The copy number and genomic location of the families vary in different strains of one species of wild rice, suggesting that these elements may still be active. Retrotransposons were also screened for by amplification of the reverse transcriptase coding region using the polymerase chain reaction (PCR). At least two types of copia-like elements (Tos4 and Tos5) were found. The total copy number of retrotransposons in the rice genome was estimated to be about 1000. These results suggest that, as in Drosophila, retrotransposons are the major transposon class in rice.
Variation of seed α-amylase inhibitors was investigated in 1 154 cultivated and 726 non-cultivated (wild and weedy) accessions of the common bean, Phaseolus vulgaris L. Four α-amylase inhibitor types were recognized based on the inhibtion by seed extracts of the activities of porcine pancreatic α-amylase and larval α-amylase and larval α-amylase of the Mexican bean weevil, Zabrotes subfasciatus Boheman. Of the 1 880 accessions examined most (1 734) were able to inhibit porcine pancreatic α-amylase activity, but were inactive against the Z. subfasciatus larval α-amylase; 41 inhibited only the larval α-amylase activity, 52 inhibited the activities of the two α-amylases, and 53 did not inhibit the activity of either of the α-amylases. The four different inhibitor types were designated as αAI-1, αAI2, αAI-3, and αAI-0, respectively. These four inhibitor types were identified by the banding patterns of seed glycoproteins in the range of 14-20 kDa by using SDSpolyacrylamide gel electrophoresis. Additionally, four different banding patterns were recognized in accessions with αAI-1, and were designated as αAI-1a, 1b, 1c, and 1d. Two different patterns of the accessions lacking an α-amylase inhibitory activity were identified and designated as αAI-0a and αAI-0b. The largest diversity for seed α-amylase inhibitors was observed in non-cultivated accessions collected from Mexico where all eight inhibitor types were detected. The possible relationships between the variation of seed α-amylase inhibitors and bruchid resistance are discussed.
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