System analysis of Phycomyces light-growth response: double mutants.

The light-growth response of Phycomyces has been studied with Gaussian white-noise test stimuli for a set of 21 double mutants affected in all pairwise combinations of genes madA to madG; these genes are associated with phototropism, the light-growth response, and other behaviors. The input-output relations of the light-growth responses of these mutants are represented by Wiener kernels in the time domain and transfer functions in the frequency domain. The results have been analyzed comparatively with those in the preceding papers on wild-type and single mutant strains. Two of the double night-blind mutants (combinations AB and BC) have especially weak, but still detectable, responses. To evaluate possible dynamic interactions among the seven mad gene products, each double-mutant transfer function was analyzed jointly with those of the parental single mutants and wild-type. Specifically, a hypothesis of dynamic independence was rejected at the 5% significance level for the following combinations: AD, AE, AG, BC, BD, BE, BF, BG, CD, CE, CF, DE, DG, and EF. A formal pictorial scheme summarizes the dynamic interactions among the mad gene products, according to this test. The high degree of interactions between the "input" gene products (A, B, and C) and the "output" gene products (D, E, F, and G) suggest that most or all of the sensory transduction pathway for the light-growth response (and phototropism) is contained in a multimolecular complex.