Differential stability of expression of similarly specified endogenous and exogenous genes in the sea urchin embryo Available to Purchase

The object of these experiments was to determine whether competitive titration in vivo of factors required for expression of the Cyllla · CAT fusion gene would affect expression of the endogenous Cyllla gene in the same embryos. Earlier work showed that expression of this fusion gene after injection into sea urchin eggs is stoichiometrically reduced when low molar excesses of DNA fragments containing only its regulatory domain are coinjected. In order to compare endogenous (i.e. Cyllla) and exogenous (i.e. Cyllla · CAT) expression simultaneously in embryos bearing excess competitor regulatory DNA, we developed, and here describe, a new procedure for generating transgenic sea urchin embryos in which all of the cells in many embryos, and most in others, bear the exogenous DNA. Such large reduction of mosaicism can be achieved by multiple injection of the exogenous DNA fragments into fertilized eggs. Using this method, we demonstrate that at a level of competitor DNA incorporation which reduces Cyllla·CAT expression by 85%, endogenous Cyllla mRNA levels are wholly unaffected. Nor is spatial expression of the endogenous Cyllla gene disturbed. Since the Cyllla · CAT genes are properly expressed under control of the Cyllla regulatory sequences, they must participate in the same set of necessary DNA-protein interactions. However, we infer from the results that we report here that the regulatory complexes in the endogenous Cyllla gene are greatly stabilized relative to those of the exogenous Cyllla · CAT genes.