CYTOPLASMIC ACTIN A3-GENE PROMOTER INJECTED AS SUPERCOILED PLASMID IS TRANSIENTLY ACTIVE IN BOMBYX-MORI EMBRYONIC VITELLOPHAGES

Freshly deposited eggs of Bombyx mori were microinjected with supercoiled plasmid DNA which carried the beta-galactosidase coding sequence of Escherichia coli inserted in place of the coding sequence of the B. mori cytoplasmic actin A3 gene. Transient expression of this fusion gene in the embryo was determined by in situ histochemical detection of enzyme activity. After injection of the plasmid at different stages of embryonic development, beta-galactosidase activity depending on the injected DNA was only detected in the vitellophages. This indicates the presence of active transactivators of the actin A3 gene promoter in this cell type. Tissue specificity of the fusion gene expression could be related to the early polyploidization of vitellophages, a process which would favour the stability of the nuclear pool of injected plasmids. The activity of the transgene in vitellophages was detectable at 24-33 h of egg development, the stage presumed for the onset of zygotic gene expression, up to the end of embryogenesis. This gene transfer system is thus promising to analyse the cis regulatory sequences of the actin A3 gene and could be utilized for other ubiquitous genes.