DIFFERENTIAL EXPRESSION OF THE MULTIGENE FAMILY ENCODING THE SOYBEAN MITOCHONDRIAL ALTERNATIVE OXIDASE

The alternative oxidase (AOX) of the soybean (Glycine max L.) inner mi tochondrial membrane is encoded by a multigene family (Aox) with three known members. Here, the Aox2 and Aox3 primary translation products, deduced from cDNA analysis, were found to be 38.1 and 36.4 kD, respect ively. Direct N-terminal sequencing of partially purified AOX from cot yledons demonstrates that the mature proteins are 31.8 and 31.6 kD, re spectively, implying that processing occurs upon import of these prote ins into the mitochondrion. Sequence comparisons show that the process ing of plant AOX proteins occurs at a characteristic site and that the AOX2 and AOX3 proteins are more similar to one another than to other AOX proteins, including soybean AOX1. Transcript analysis using a poly merase chain reaction-based assay in conjunction with immunoblot exper iments indicates that soybean Aox genes are differentially expressed i n a tissue-dependent manner. Moreover, the relative abundance of both Aox2 transcripts and protein in cotyledons increase upon greening of d ark-grown seedlings. These results comprehensively explain the multipl e AOX-banding patterns observed on immunoblots of mitochondrial protei ns isolated from various soybean tissues by matching protein bands wit h gene products.