Sucrose synthase (SuSy) is a key enzyme which catalyzes the reversible cleavage of phloem-transported sucrose. To obtain a SuSy cDNA for mapping, we screened a rice cDNA library from cv. 'Nipponbare' using a PCR product obtained on the basis of the coding region sequence of rice SuSy gene 1 (RSs1) (accession: X64770). A cDNA consisting of 2.4 kbp (clone name: fss1)was isolated, whose 3'end was the polyA signal region. The fss1 sequence is identical to the 2.4 kbp region of the RSs1 (Accession:mRNA Z15028;2.6kbp, including the ORF). Two portions of fss1 were used as probes for RFLP analysis:the 600bp fragment of the 5' end region of the clone (probe A), and the 1.8 kbp fragment of the 3' region of the clone (probe B). RFLP analysis in this study was carried out according to the method of Saito et al. (1991). The autoradiogram with probe A showed that the HindIII-digested DNAs produced polymorphic major bands with sizes of 2.9 kbp in 'Kasalath' lane and 2.6 kbp in 'FL134' lane, with a monomorphic minor band (3.2 kbp) in each lane. The HindIIIdigested DNAs of the F₂ plants were used for the F₂ analysis using probe A. The segregation ratio was as follows: 'Kasalath' homozygotes : heterozygotes : 'FL134' homozygotes = 35 : 75 : 25 (chi² = 3.15, P>0.05). This locus was located between XNpb165-1 and XNpb200, on chromosome 6 (tentative locus name: XcrSsA, Fig. 1). The autoradiogram with probe B showed that the EcoRV-digested DNAs produced several monomorphic bands in both parental lanes and the only null allelic band with size of 11 kbp in 'Kasalath' lane. The EcoRV- digested DNAs of the F₂ plants were used for the F₂ analysis using probe B. The segregation ratio was as follows: 'Kasalath' homozygotes and heterozygotes : 'FL134' homozygotes = 103 : 37 (chi² = 0.15, P>0.05). This locus was located between XNpb117 and XNpb379, on chromosome 7 (tentative locus name: XcrSsB, Fig. 1). We estimated the copy number of sequence homologous to RSs1 to be 3 copies per genome on the basis of the autoradiograms mentioned above, supporting the result by Huang et al. (1996), who isolated 3 SuSy genes. To confirm that these two SuSy loci represent two of the 3 rice SuSy loci, we compared the previously-reported map positions for SuSy loci among Poaceae crops. We confirmed that XcrSsA on chromosome 6 was the RSs1 locus, which was identical to R1966, previously-mapped locus by the Rice Genome Research Program (http://rgp.dna.affrc.go.jp/publicdata/geneticmap2000/chr06.html), and that XcrSsB on chromosome 7 should be RSs3, the rice locus orthologous to wheat XSs2 and barley XSs2 (Fig. 1). These orthologus relations were supported by the comparison data of DNA sequences and deduced amino acid sequence for SuSy genes of Poaceae crops. So far we have not found a sequence perfectly matched with the DNA sequcence of RSs3 (Accession:L03366) by BLASTN

"Choose databes = nr", except L03366 itself. On the other hand, our result by BLASTN "Choose database = nr" using the RSs2 sequence (the gene: accession X59046) showed that the RSs2 sequence was perfectly matched with a part of the sequence in a genomic clone from rice chromosome 3 (Accession: AC084380).

References

Causse, M.A., Fulton, T.M., Cho, Y.G., Ahn, S.N., Chunwongse, J., Wu, K., Xiao, J., Yu, Z., Ronald, P.C., Harrington, S.E., Second, G., McCouch, S. R. and Tanksley, S.D. (1994). Saturated molecular map of the rice genome based on an interspecific backcross population. Genetics 138, 1251-1274.

Huang, J.-W., Chen, J.-T., Yu, W.-P., Shyur, L.-F., Wang, A.-Y., Sung, H.-Y., Lee, P.-D. and Su, J. -C. (1996). Complete structures of three rice sucrose synthase isogenes and differential regulation of their expressions. Biosci. Biotech. Biochem. 60, 233-239.

Kurata, N., Moore, G., Nagamura, Y., Foote, T., Yano, M., Minobe, Y. and Gale, M. (1994). Conservation of genome structure between rice and wheat. Bio/Technology 12, 276-278.

Saito, A., Yano, M., Kishimoto, N., Nakagahra, M., Yoshimura, A., Saito, K., Kuhara, S., Ukai, Y., Kawase, M., Nagamine, T., Yoshimura, S., Ideta, O., Ohsawa, R., Hayano, Y., Iwata, N. and Sugiura, M. (1991). Linkage map of restriction fragment length polymorphism loci in rice. Jpn. J. Breed. 41, 665-670.

Sanchez de la Hoz, P., Vicente-Carbajosa, J., Mena, M. and Carbonero, P. (1992). Homologous sucrose synthase genes in barley (Hordeum vulgare) are located in chromosome 7H (syn. 1) and 2H. FEBS lett. 310, 46-50.