Changes in ribonuclease activity during rust infection I. Characterization of multiple molecular forms of ribonuclease from flax rust grown in host-free media

Abstract

Sephadex G100 gel filtration of cell-free extracts from flax rust ( Melampsora lini (Ehrenb.) Lev., race 3) uredospores as well as mycelium grown in axenic culture yields three molecular forms of ribonuclease (RNase). These are referred to as M-I, M-II and M-III according to their order of elution; their apparent molecular weights are 75 000, 43 000 and 18 000 respectively. Flax rust mycelium growing on partially defined and completely defined solid and liquid media also secretes an extramycelial RNase (M-IV), with a molecular weight of 32 000. These four RNases are remarkably different from one another with respect to the following properties: (a) pH optima, (b) K m and V max, (c) effects of Mg 2+, EDTA and urea, (d) heat-stability and (e) specificity for bases at the cleavage site. These results permit the conclusion that the multiple forms of flax rust RNase are structurally dissimilar. Furthermore, their catalytic properties and the observed effects of various reagents on their activity provide circumstantial evidence that each of the three high molecular weight species, M-I, M-II and M-IV, is composed of more than one polypeptide component. To our knowledge, this is the first report on the characterization of RNase from a rust fungus cultured on host-free media.