Alcohol-soluble proteins, prolamins, comprise about 18 to 20% of the total endosperm protein in rice. Prolamins are classified into 10, 13, 15 and 16 kD polypeptide groups, each of them is composed of several components with different isoelectric points (Hibino et al. 1989). The number and staining intensity of prolamin polypeptide bands greatly differ among cultivars from different countries (Satoh et al. 1990, Minakuchi et al. 1994). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing gel electrophoresis (IEF) are useful not only for revealing variations but also for identification of a variety in seed storage proteins. Seed storage proteins are primary gene products, therefore, the variation in protein composition corresponds to genotypic variation. This paper deals with the genetic variation of a seed storage protein, prolamin, in Myanmar local rice cultivars.

A total of 350 local rice cultivars collected from seven distinct regions of Myanmar were obtained from the Seed Bank of Myanmar. These cultivars were analyzed by SDS-PAGE and IEF. Japonica rice Kinmaze and indica rice IR36 were used as controls. Prolamins were extracted with 5% 2-mercaptoethanol and 60% n-propanol; SDS-PAGE was carried out using the discontinuous buffer system of Laemmli (1970).

Prolamins were separated into 16, 15, 13 and 10 kD polypeptide groups by SDS-PAGE. Prolamins from Myanmar local rice cultivars widely varied in the presence or absence, and staining intensity of their respective polypeptide bands. Kinmaze and IR36 showed similar SDS-PAGE profiles. Myanmar local cultivars were divided into at least seven types of prolamins based on SDS-PAGE profiles (Fig. 1). Lane 1 showed 14 kD band present type. Lane 2 showed 13 kD and 15 kD bands with high intensity, which was identical to Kinmaze and IR36. Lane 3 showed 12.5 kD band present type. Lane 4 showed 13 kD band with high intensity, and 15 kD band with low intensity. Lane 5 showed the presence of 14 kD band and high intensity 15 kD band. Lane 6 showed both 13 and 15kD band, with low intensity. Lane 7 showed 13 kD band absent type.

IEF analysis was carried out because prolamins varied in their electric charges and isoelectric point (pI), in addition to molecular mass (M. M). IEF gels were prepared according to the modified method of Uemura et al. (1996). pI 7.1 and 6.95 bands were specific to Kinmaze; pI 6.90 and 6.35 bands were specific to IR 36. The SDS-PAGE pattern of prolamins extracted

from Kinmaze was similar to that of IR36, while the IEF band patterns of prolamins in both cultivars were different. A great variation in prolamin was observed not only in pI but also in the number and staining intensity of IEF bands of these accessions under investigation. Myanmar local cultivars were classified into at least 36 types, based on variation in prolamins, by IEF (Fig. 2). IEF band patterns of prolamin in all of the Myanmar local cultivars were different from those of Kinmaze and IR36. Each of prolamin types by SDS-PAGE showed a wide variation in pI, the number and staining intensity of IEF bands, suggesting that SDSPAGE and IEF variations are not strongly correlated. These results indicate that Myanmar local rice cultivars are an important source to improve the ploramin.

References

Hibino, T., K. Kidzu, T. Masumura, K. Ohtsuki, K. Tanaka, M. Kawabata and S. Fuji, 1989. Amino acid composition of rice prolamin polypeptides. Agric. Biol. Chem. 53 : 513-518.

Laemmli, U. K, 1970. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227: 680-685.

Minakuchi, S., H. Satoh and T. Konishi, 1994. Polymorphism of prolamin polypeptides in rice cultivars from Bangladesh, Nepal and Bhutan, a preliminary study. RGN 11: 175-177.

Satoh, H., X. R. Rakotonjanahary and T. C. Katayama, 1990. SDS-PAGE analysis of storage proteins of cultivated rice collected in Madagascar, 1988. Kagoshima Univ. Res. Center S. Pac., Occ. Papers. 18: 114-126.

Uemura, Y. J, Satoh, H., Ogawa, M., Suehisa, H., Katayama, T. C and Yoshimura, A, 1996. Chromosomal location of genes encoding glutelin polypeptides in rice. In Rice Genetics III, International Rice Research Institute, Manila, Philippines, pp 471-476.