A Nuclear Mutant of Chlamydomonas reinhardtii Defective in Photosynthetic Photophosphorylation

Use of the Flagyl selection procedure [Schmidt et al. (1977) Proc. Natl Acad. Sci. USA, 74 , 610‐614] led to the isolation of a nuclear mutant of Chlamydomonas reinhardtii designated thm‐24. This mutant displays normal electron transport rates in vitro , possesses high latent ATPase activity bound to the thylakoid membrane, but is incapable of photophosphorylation. Decay of the transmembrane potential, as indicated by the kinetics of the 520‐nm absorption change after illumination, is unusually slow and markedly biphasic. Sodium dodecylsulfate/polyacrylamide gel electrophoresis of purified thylakoid membrane. shows mutant thm‐24 to be lacking a number of polypeptides including those previously designated 4.1, 4.2 and 8.1. Treatment of purified thylakoid membranes of wild‐type and mutant algae, using the chloroform‐release procedure of Beechey et al. [(1975) Biochem. J. 748 , 533–537] resulted in the removal of ATPase activity from each strain. In wild‐type cells, the ATPase activity was of heterogeneous enzymatic origin; fractionation of the chloroform‐release extracts by non‐denaturing polyacrylamide gel electrophoresis yielded three distinct bands displaying ATPase activity, designated ATPases I, II and III. In contrast, extracts from membranes of mutant thm‐24 yielded only one ATPase‐containing fraction, co‐migrating with ATPase I from wild‐type. Use of electrophoretic, immunological and enzymatic methods established a correspondence of the polypeptide subunits of ATPases II and III and those of spinach coupling factor, CF 1 . ATPase I from either algal strain was shown to be structurally distinct from higher plant CF 1 and to C. reinhardtii ATPases II and III.