TRUNCATED PRESEQUENCES OF MITOCHONDRIAL F1-ATPASE BETA-SUBUNIT FROM NICOTIANA-PLUMBAGINIFOLIA TRANSPORT CAT AND GUS PROTEINS INTO MITOCHONDRIA OF TRANSGENIC TOBACCO

The mitochondrial F-1-ATPase beta subunit (ATPase-beta) of Nicotiana p lumbaginifolia is nucleus-encoded as a precursor containing an NH2-ter minal extension. By sequencing the mature N. tabacum ATPase-beta, we d etermined the length of the presequence, viz. 54 residues. To define t he essential regions of this presequence, we produced a series of 3' d eletions in the sequence coding for the 90 NH2-terminal residues of AT Pase-beta. The truncated sequences were fused with the chloramphenicol acetyl transferase (cat) and beta-glucuronidase (gus) genes and intro duced into tobacco plants. From the observed distribution of CAT and G US activity in the plant cells, we conclude that the first 23 amino-ac id residues of ATPase-beta remain capable of specifically targeting re porter proteins into mitochondria. Immunodetection in transgenic plant s and in vitro import experiments with various CAT fusion proteins sho w that the precursors are processed at the expected cleavage site but also at a cryptic site located in the linker region between the preseq uence and the first methionine of native CAT.