Low variability of the protein species synthesized byDrosophila melanogaster embryos

Stainable proteins as well as newly synthesized polypeptide chains of proteins extracted fromDrosophila melanogaster embryos were analyzed by two-dimensional gel electrophoresis. The following developmental stages were studied: unfertilized eggs, early nuclear multiplication (25 min average age), late nuclear multiplication (105 min), cellular blastoderm (165 min), gastrula (4 h), mesodermal segmentation (6 h) and muscleattachment (8 h). One hundred and fifty stainable spots were present at all developmental stages and were all also synthesized during development, with the exception of 5 unkown proteins and the three yolk proteins. Out of 400 proteins which were labelled by³⁵S-methionine, only 5% showed a reproducible pattern of variable synthesis. Three proteins appeared upon fertilization. The early nuclear multiplication stage showed the largest number of labelled spots while the lowest number of labelled spots was observed at blastoderm formation. The pattern of synthesis of a few specific proteins was also followed. Actin I was synthesized only at 8 h, actin II and actin III were synthesized at all stages. β-tubulin was synthesized at all stages, while we observed a reduction, if not a cessation, of synthesis of α-tubulin at 105 min, 165 min and 4 h of development. Non heat-shock embryos synthesized a large amount of heat-shock protein (hsp) 84 at 25 min while hsp 70 and 68 were first detected after 4 h of development. Though it is generally accepted that the embryonic genome is activated at blastoderm formation we did not observe a parallel increase in protein species. It is possible that protein synthesis on the new transcripts is below the detection limit of the technique. Alternatively the embryonic messages may gradually substitute preexisting maternal messages or only become available for translation some time after they are transcribed.