Molec. gen. Genet. 159, 85-87 (1978) © by Springer-Verlag 1978 lmaginal Disc Ribosomal Proteins of D. melanogaster* lffva Fekete Institute of Genetics, Biological Research Center of the Hungarian Academy of Sciences, P.O.B. 521, H-6701 Szeged, Hungary Andrew G. Lambertsson** Department of Genetics, University of Ume~, S-90187 Umegt, Sweden Summary. The ribosomal proteins from undifferen- tiated imaginal discs of Drosophila melanogaster were analyzed by two-dimensional gel electrophoresis and compared with the ribosomal protein pattern of adult flies. It is shown that the ribosomal proteins from these discs are qualitatively identical with those of adult flies except that two acidic proteins are missing in the discs. This heterogeneity is discussed in terms of the functional roles these two proteins may carry in connection with disc differentiation. Introduction An important and characteristic feature of the devel- opment of holometabolous insects such as Drosophila is the establishment, within one individual, of two systems, a larval and an imaginal one. The former (the epidermis, the salivary glands, the fat bodies, the intestine, and the muscles) is programmed to de- generate, and the latter (the imaginal discs) to un- dergo rapid and extensive differentiation, forming the adult structures. The imaginal discs are derived from the epidermis during embryogenesis and are not of vital importance before the pupal stage. The number of cells in the discs remains stationary during embryo- genesis, but then increases continuously and exponen- tially throughout the larval period (N6thiger, 1972). Under the influence of ecdysone, the steroid insect molting hormone, the discs undergo morphogenesis. The initial phase of this process is termed evagination and is dependent on continued protein synthesis (Fris- trom et al., 1973). Previous studies, using whole animals, disclosed qualitative changes in the protein composition of Dro- * Paper No. 6 of the present series. Paper No. 5 is by Lamberts- son, A.G. Molec. gen. Genet. 139, 145-156 (1975) ** To whom all correspondence should be sent sophila ribosomes during development (Lambertsson et al., 1970; Lambertsson, 1972, 1974), the alterations taking place during the third larval instar (Lamberts- son, 1975). In view of these findings, and the observa- tion that the cells of mature discs are packed full with free ribosomes (Ursprung, 1972), it was of inter- est to know the ribosomal protein composition of undifferentiated imaginal discs. The results presented in this paper demonstrate that the ribosomal proteins of imaginal discs and adult flies are qualitatively identical except that two acidic proteins are missing in the discs, Material and Methods The Preparative Isolation of Imaginal Discs. Imaginal discs were isolated from the Oregon R wild-type strain according to the pre- parative procedure described by Fristrom (1972). The imaginal disc fraction was estimated to have a purity of 90-95%. The discs were frozen and stored at -30 ° C until used. Preparation of Ribosomes and Ribosomal Proteins. The conditions for the preparation of high salt-washed ribosomes were the same as described before (Lambertsson, 1974) with the following modifi- cations: a) imaginaI discs (1 1.5 g, wet weight) were homogenized in 5 ml of buffer A (0.25 M sucrose, 0.05 M Tris-HCl, pH 7.6, 0.01 M MgCI2, 0.05 M NH,C1, 0.01 M mercaptoethanol, and 1% Triton X-100), using a Dounce all-glass homogenizer, and b) 4 ml of the mitochondria-free supematant was underlayed with 1 ml of buffer B (1.0 M sucrose, 0.05 M Tris-HC1, pH 7.6, 0.01 M MgC12, 0.50 M NH4CI, and 0.02 M mercaptoethanol), and the ribosomes were pelleted by ultracentrifugation for 4 h at 160000 x g. The ribosomal proteins were extracted with 67% acetic acid in the presence of 33 mM Mg~ + (Hardy et al., 1969; Lambertsson, 1975). The proteins were precipitated with 9 volumes of cold ace- tone, washed twice with acetone, and dried in vaeuo (Lambertsson et al., 1970). Two-Dimensional Electrophoresis. Two-dimensional polyacryl- amide gel electrophoresis was performed as previously described (Lambertsson, 1975), except that the 2-D apparatus was modified 0026-8925/78/0159/0085/$01.00