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

The white-noise method of system identification has been applied to the transient light-growth response of a set of seven mutants of Phycomyces with abnormal phototropism, affected in genes madA to madG. The Wiener kernels, which represent the input-output relation of the light-growth response, have been evaluated for each of these mutants and the wild-type strain at a log-mean blue-light intensity of 0.1 W m-2. Additional experiments were done at 3 X 10(-4) and 10 W m-2 on the madA strain C21 and wild-type. In the normal intensity range (0.1 W m-2) the madA mutant behaves similarly to wild-type, but, at high intensity, the madA response is about twice as strong as that of wild-type. Except for C21 (madA), the first-order kernels of all mutants were smaller than the wild-type kernel. The first-order kernels for C111 (madB) and L15 (madC) show a prolonged time course, and C111 has a longer latency. The kernels for C110 (madE), C316 (madF), and C307 (madG) have a shallow and extended negative phase. For C68 (madD), the latency and time course are shorter than in the wild-type. These features are also reflected in the parameters estimated from fits of the analytical model introduced in the previous paper to the experimental transfer functions (Fourier transforms of the kernels). The kernel for L15 (madC) is described better by a model that lacks one of the two second-order low-pass filters, because its response kinetics are dynamically of lower order.