THE PLANT MITOCHONDRIAL 22 KDA (PSST) SUBUNIT OF RESPIRATORY-CHAIN COMPLEX-I IS ENCODED BY A NUCLEAR GENE WITH ENHANCED TRANSCRIPT LEVELS IN FLOWERS

Genes for subunits of respiratory chain complex I are found in mitocho ndrial, plastid and/or nuclear genomes with varying distributions in t he diverse eukaryotic species. The intrinsic PSST subunit of complex I is a mitochondrially encoded protein in Paramecium but is specified b y a nuclear gene in animals. In plants to date only the homologous pla stid encoded NDH-K gene product has been described. The analogous plan t mitochondrial protein is now identified as the 22 kDa complex I subu nit and found to be encoded in the nuclear genome of Arabidopsis and p otato. The cDNA sequences of clones isolated from both plants are 79% identical in the conserved coding region, while the 5' parts of the re ading frames specifying the N-terminal presequences for mitochondrial import differ significantly. The expression of the genes examined in d ifferent organs of both plants by Northern blot analysis shows elevate d steady-state mRNA levels in flowers. Hence, expression of the gene a ppears to be organ-specifically regulated by its transcription rate an d/or mRNA stability. A 1.6 kb long genomic DNA sequence of Arabidopsis upstream of the transcribed gene region encoding the PSST subunit in Arabidopsis contains several putative promoter sequence motifs. The re sults are discussed with regard to the appearance of a nuclearly integ rated, former mitochondrial gene.