Leaf senescence in a non-yellowing mutant of Festuca pratensis : II. Proteolytic degradation of thylakoid and stroma polypeptides.

The acid proteinases (E.C. 3.4.4.?) of mature, nonsenescent Festuca pratensis leaf tissue were fractionated into one major and three minor activities by ion exchange chromatography. During senescence of leaf sections in darkness there was a general increase in total proteinase activity but no qualitative alteration in the proteinase complement. The pattern of proteinases in the nonyellowing mutant Bf 993 was essentially identical to that of the normal genotype Rossa. The pH responses of proteinases from senescent Bf 993 and Rossa tissue were measured using as substrates (14)C-labelled protein fractions prepared from barley seedlings. These fractions were: ribulose bisphosphate carboxylase; low molecular weight soluble protein; ethylene diamine tetraacetate soluble thylakoid protein; chloroform: methanol soluble thylakoid protein; and residual protein. For each substrate, the pH optimum was 5-6, there was a shoulder on the neutral-alkaline side of the pH curve and hydrolase activity was stimulated by borate at high pH. The responses of Bf 993 proteinases were very similar to those of Rossa. These results suggest that it is the accessibility of thylakoid proteins to proteinase action rather than proteinase per se that is impaired in the non-yellowing mutant.