Figure 5.

Scheme illustrating the hypothesis presented in this paper for the evolution of the modern CPS synthetase subunit from a dimeric CK (K). The nitrogen donor (NH₃) is not represented nor the glutaminase subunit which, in a later step, is assumed to have combined with the synthetase subunit, conferring upon the enzyme the ability to use glutamine as nitrogen donor. During the transition between steps II and III, fusion (or recombination) joining the primeval synthetase gene (K′) may have preceded divergent evolution of the two copies of the primeval synthetase gene (K′) toward subdomains CPS.A and CPS.B.