System analysis of Phycomyces light-growth response. Photoreceptor and hypertropic mutants.

The light-growth responses of Phycomyces behavioral mutants, defective in genes madB, madC, and madH, were studied with the sum-of-sinusoids method of system identification. Modified phototropic action spectra of these mutants have indicated that they have altered photoreceptors (P. Galland and E.D. Lipson, 1985, Photochem. Photobiol. 41:331). In the two preceding papers, a kinetic model of the light-growth response system was developed and applied to wild-type frequency kernels at several wavelengths and temperatures. The present mutant studies were conducted at wavelength 477 nm. The log-mean intensity was 6 X 10(-2)W m-2 for the madB and madC night-blind mutants, and 10(-4)W m-2 for the madH hypertropic mutant. The prolonged light-growth responses of the madB and madC mutants are reflected in the reduced dynamic order of their frequency kernels. The linear response of the hypertropic mutant is essentially normal, but its nonlinear behavior shows modified dynamics. The behavior of these mutants can be accounted for by suitable modifications of the parametric model of the system. These modifications together support the hypothesis that an integrated complex mediates sensory transduction in the light responses and other responses of the sporangiophore.