41. Copy number of RMu elements in Japanese lowland rice landrace
Y. 
MIYAsHITA’, R. ISHIKAWA1, M. SENDA2, S. AKADA2, T. HARADA’ and M. NIIZEKI1
1) 
Faculty of Agriculture and Life Science, Hirosaki Univercity, Hirosaki, 036-8561 Japan
2) 
Gene Research Center, Hirosaki Univercity, Hirosaki, 036-8224 Japan
     Rice Mutator (RMu) transposable element is a homolog of maize Mutator transposon which is well known to be used to tag genes in maize genome (Bennetzen eta!. 1993). In this report, we examined variation of copy numbers of RMu element in Japanese cultivars to study transposition events which may have happened in the past. This information would be useful in searching for currently active elements. To estimate copy number of RMu elements, we used two probes named as TNP and TIR. TNP is a PstI-XhoI fragment of an orf in RMu1-IR36, carrying a conservative putative transposase domain abbreviated as TNP domain, (Ishikawa and Freeling in press). TIR is 3’ sequences of RMu1-1R36 including a 193bp right termini. We counted number of bands detected in southern blots as copy number in rice genome when genomic DNA was digested with SadI and probed with TIR or TNP, because SadI does not cut inside of RMu elements ever cloned (data not shown). TNP was used to count copy number of elements in RMu1 class carrying TNP domain and TIR to count additional copies in RMu2 class which has no TNP domain but homologous termini.
     Genomic DNAs of 53 lowland rice strains were digested with SadI and firstly probed with TIR (Fig. 1A). Based on the Southern blot analysis, banding patterns revealed by the 53 strains were classified into six types. Copy numbers of these six types varied from four to five.Type 1 is composed of 11 strains and a representative strain is Akamai uruchi. Type 4 is composed of 36 strains and a representative strain is Akamochi. Three strains, Mizukuchimochi, Aikoku, and Airyoushaku compose Type 5. Type 6 is Mizukuchime. Type 7 Kaori-ine. Type 8 Nagoakaho.
     Types 2 and 3 are only found in Japanese upland rice strains (data not shown). The blots used with the above experiment were next hybridized with a TNP probe. Eleven strains belong to Type 1 were also grouped into two types. A pattern of the ten strains was shown in lane 1 of Fig. 1 B and that of the remaining one strain (Karahoushi) was shown in lane 2 of the same figure. These observations suggest that Karahoushi would gain one copy of RMu1 class per haploid. These results further suggest that a RMu element transposed, at least in the past. Based on these data, RMu elements in lowland rice landraces seem to be inactive in general. However high activity has been found in other strains (Ishikawa et al. in this volume). The difference in the activity may have resulted from their cultivation conditions or genetic background of respective strains.

Reference

Bennetzen, j.l. P.S. Springer, A.D. Cresse and M. Hendrickx, 1993. Specificity and regulation of the Mutator transposable element system in maize. Critical Reviews in Plant Sience 12: 57-95.