Actin synthesis during the early development of Xenopus laevis

Cytoskeletal beta and gamma-actin are amongst the most rapidly made proteins of oocytes, blastulae and late embryonic stages of Xenopus laevis but, relative to other proteins, the rate of synthesis is low in the egg or cleaving embryo, although the messenger RNA is present in comparable amounts at the different stages. Actin synthesis therefore involves post-transcriptional regulation. alpha-Actin, the actin class characteristics of straited muscle cells, is first detectable in late gastrulae and it is an abundant newly synthetized protein from the neurula stage onwards. mRNA template activity for this protein is not detectable before the gastrula stage. Thus alpha-actin synthesis probably reflects new gene action, confined to part of the embryo, for alpha-actin only appears in the section which includes presumptive skeletal muscle cells. It therefore constitutes the earliest cyto-specific protein so far demonstrated in Amphibia. When tadpole tail poly(A)-containing mRNA is injected into oocytes and eggs alpha-actin synthesis is seen in both cases. Extensive evidence for the identification of the actins is presented. This is based on location of synthesis, DNase-I binding and partial peptide mapping.