The object of these experiments was to determine whether competitive titration in vivo of factors required for expression of the CyIIIa·CAT fusion gene would affect expression of the endogenous CyIIIa 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. CyIIIa) and exogenous (i.e. CyIIIa·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 CyIIIa·CAT expression by 85%, endogenous CyIIIa mRNA levels are wholly unaffected. Nor is spatial expression of the endogenous CyIIIa gene disturbed. Since the CyIIIa·CAT genes are properly expressed under control of the CyIIIa 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 CyIIIa gene are greatly stabilized relative to those of the exogenous CyIIIa·CAT genes
