Simplest model that fits the data for information flow during DspA signalling. Acetyl phosphate and DspA can both act as phosphodonors to one or more response regulators. Under nitrogen starvation, phosphorylation by either acetyl phosphate or DspA alone is sufficient to saturate the response. Thus pta or dspA single mutants behave like the wild-type, and only the pta-dspA- double mutant has a distinctive phenotype. Under other conditions the steady-state phosphorylation level of the response regulators (or hybrid kinases) is not saturated either by acetyl phosphate or by DspA alone. Then pta and dspA deletions have additive effects. Both single mutants show a phenotypic change, but a stronger effect is seen in the double mutant. RR: response regulator; AK: acetate kinase; PTA: phosphotransacetylase.