DELETION OF THE STRUCTURAL GENE FOR THE NADH-DEHYDROGENASE SUBUNIT-4 OF SYNECHOCYSTIS-6803 ALTERS RESPIRATORY PROPERTIES

Chloroplasts and cyanobacteria contain genes encoding polypeptides hom ologous to some subunits of the mitochondrial respiratory NADH-ubiquin ol oxidoreductase complex (NADH dehydrogenase). Nothing is known of th e role of the NADH dehydrogenase complex in photosynthesis, respiratio n, or other functions in chloroplasts, and little is known about the s pecific roles of the perhaps 42 subunits of this complex in the mitoch ondrion. Inactivation of a gene for subunit 4 (ndhD-2, ndh4) of this c omplex in the cyanobacterium Synechocystis 6803 has no effect on photo synthesis, judging from the rate of photoautotrophic growth of mutant cells, but the mutant's respiratory rate is about 6 times greater than that of wild-type cells. Respiratory electron transport activity in c yanobacteria is associated both with photosynthetic thylakoid membrane s and with the outer cytoplasmic membrane of the cell. Cytoplasmic mem branes of mutant cells have much greater NADH-dependent cytochrome red uctase activity than preparations from wild-type cells; this activity remains at wild-type levels in isolated thylakoid membranes. It is sug gested that the 56.6-kD product of ndhD-2 is not essential for the act ivity of a cytoplasmic membrane-bound NADH dehydrogenase but that it r egulates the rate of electron flow through the complex, establishing a link between this ndh gene and respiration. The activity of the molec ularly distinct thylakoid-bound NADH dehydrogenase is apparently unaff ected by the loss of ndhD-2.