The proteins of the "Caulobacter" cell cycle control system are widely co-conserved across the alphaproteobacteria, but the ultimate function of this regulatory system varies widely in different species. These evolutionary changes reflect enormous differences between the individual species in fitness strategies and ecological niches. For example, "Agrobacterium tumefaciens" is a plant pathogen, "Brucella abortus" is an animal pathogen, and "Sinorhizobium meliloti" is a soil bacterium that invades, and becomes a symbiont in, plant root nodules that fix nitrogen yet most of the proteins of the "Caulobacter" cell cycle control are also found in these species. The specific coupling between the protein components of the cell cycle control network and the downstream readout of the circuit differ from species to species. The pattern is that the internal functionality of the network circuitry is conserved, but the coupling at the “edges” of the regulatory apparatus to the proteins controlling specific cellular functions differs widely among the different species.