Reduction of mRNA levels for the 28.5 kDa iron-sulfur core protein of mitochondrial complex I inhibits pollen maturation in transgenic tobacco

The multi-subunit enzyme complex I of the mitochondrial respiratory chain i s an assembly of nuclear and organellar encoded proteins with distinct role s and functions. The nuclear encoded 28.5 kDa iron-sulfur protein is locate d at the centre of electron transfer to ubiquinone. Functional importance a nd regulatory tolerance of this subunit were investigated in transgenic tob acco plants carrying antisense constructs driven by the CaMV 35S promoter. In all of the regenerated transgenics vegetative growth is undisturbed, whi le in many transformants flower development is abnormal and pollen fertilit y is reduced. Maximal observed suppression of the steady-state 28.5 kDa mRN A level reaches only about 30%. Apparently, further reduction is lethal to the vegetative tobacco plants, suggesting that the 28.5 kDa subunit is regu lated from the steady-state level onwards with little tolerance and no addi tional possibilities for compensation. This contrasts with the higher flexi bility of the NADH-binding subunit of complex I, which vegetatively survive s a 70% reduction of its mRNA level.