Mechanisms of differential regulation. The C4-specific regulation of four hypothetical genes and their protein products is illustrated. During the establishment of the C4 syndrome, novel regulatory mechanisms that were absent in the C3 state control the accumulation of proteins A and B. In the case of A, a protein that accumulates in all photosynthetic cell types in the C3 state is restricted to a specific cell type, by a trans acting factor X, without any change to biochemical function. Gene B, expressed at low levels, accumulates in a specific cell type. Proteins C and D also accumulate differentially in the C4 state, but, in these instances, differential accumulation is the result of a novel cellular environment. In the case of C, protein accumulation is regulated post-translationally at the level of assembly or stability through interaction with A. In the C4 state and the absence of A, product C fails to accumulate. Such a scenario does not require differential accumulation of C transcripts, but could affect transcription of other genes. Transcription of gene D is linked, either directly of indirectly, to the presence of a metabolic signal Y. The accumulation of Y is governed by the activities of proteins A and B. In the C4 state, changes in the accumulation of A and B affect the levels of Y and subsequently alter the accumulation of D. Boxes represent genes and spheres represent gene products.