POSSIBLE ROLES OF CALCIUM and CALMODULIN IN THE LIGHT‐STIMULATION OF WALL BIOSYNTHESIS IN Phycomyces

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Abstract
Abstract— We studied the effect of light on the activity of chitin synthetase from Phycomyces blakeslee‐anus in vitro in order to gain information on the biochemical bases of the photostimulation of sporangiophore growth. We observed that chitin synthetase activity of cell‐free extracts prepared from both sporangiophores and vegetative mycelium was increased by illumination. The process was stimulated by calcium and was not affected by protease inhibitors. Iodoacetamide inhibited chitin synthetase activation by light, or by controlled proteolysis. Calcium activated chitin synthetase by two mechanisms, one independent of and one dependent on calmodulin, the latter being sensitive to trifluoperazine. Trifluoperazine also inhibits sporangiophore growth and more noticeably phototro‐pism. We suggest that illumination increases net wall growth through two light‐sensitive calcium and calcium‐calmodulin systems one of which stimulates the normal chitin synthetase activation mechanism, and another one probably involved in vesicle displacement to the growing zone of the cell.
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