Expression of Exogenous Genes in Xenopus Oocytes, Eggs, and Embryos

Molecular biology of gene expression in early amphibian embryogenesis began in 1964, when undegraded, as opposed to alkaline-hydrolyzed, PCA (perchloric acid), or TCA (trichloroacetic acid)-degraded, RNAs were extracted by Brown and Littna (1 –3 ), Shiokawa and Yamana (4 ,5 ), and Woodland and Gurdon (6 ) from amphibian embryos using phenol methods. From these early studies, especially from those of Don Brown, it was realized that amphibian embryos exhibit quite unusual RNA synthetic patterns; the pattern of active 4S RNA (mainly tRNA) synthesis with no rRNA synthesis in pregastrula stages and the pattern of gradually increasing rRNA synthesis in post-gastrular stages. It was then reported in 1982 by Newport and Kirschner (7 ,8 ) that in Xenopus embryogenesis, large changes called midblastula transition (MBT) take place at the 12th cleavage, which include the appearance of Gl phase in cell cycle, onset of gene expression from zygotic nuclei, and acquisition of cellular motility (7 ,8 ). We then found in 1987 that Xenopus embryogenesis consists of three different phases with respect to the RNA synthetic pattern. The phase of a low level (on a per-embryo but not necessarily on a per-cell basis) of mRNA synthesis (pre-MBT stage), the phase of extremely active tRNA synthesis (both on per-cell and per-embryo bases) (MBT stage), and the phase of a nearly constant level (per cell) of rRNA synthesis (post-MBT stage) (9 –12 ).