I think the presence of a signal affect the expression of the structural enzymes in that it stops the production of the structural enzymes. Without the signals, the structural enzymes are produced. Genomic DNA contains both the structural genes that encodes products that serve as cellular structures or enzymes, and regulatory genes, which encode products that regulate the expression. In multicellular organisms allows for cellular differentiation while in single celled organisms like prokaryotes , regulation ensures that a cell's resources are not wasted making proteins that the cell does not need at that time.