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The presence of plastids in plant cells requires a higher level of precursor recognition by the mitochondrial protein import apparatus than in nonplant organisms. Although the plant presequences display the overall features observed in yeast and mammals, they are generally longer and more hydrophilic. Most of them are highly organelle specific, but some have ambiguous targeting specificity delive-ring a protein to both mitochondria and chloroplasts. Many components of plant protein import apparatus appear different to that in yeast and mammalian systems. The three outer membrane mitochondrial proteins characterized to play role as receptors in plants – Tom20, OM64, and metaxin – are plant specific. However, the channel forming units of the TOM and SAM complexes, Tom40 and Sam50, respectively, are orthologous to these components in yeast. While components of the MIA and TIM complexes also display high levels of orthology, functional studies indicate divergences in function and mechanism. Differences exist also in terms of intraorganellar localization of proteolytic events, e.g., the location of the mitochondrial processing peptidase, MPP, involved in removing targeting signals is different, whereas the function and location of the presequence protease, PreP, degrading targeting peptides, is well conserved. Overall, although the protein import machinery of mitochondria from all organisms appears to have coopted and uses the channel forming subunits from the endosymbiont that gave rise to mitochondria, there is a greater diversity in plant components in comparison to those from nonplant species. [ABSTRACT FROM AUTHOR]

Abstract The plant mitochondrial genetic system has to provide about 35 -polypeptides, which are essential for cell survival as they are components of the respiratory chain or contribute to its biogenesis. Thus, an active mitochondrial translation system is an absolute requisite. With the exception of a few ribosomal proteins, all protein factors involved in plant mitochondrial translation are nuclear-encoded and imported via the classical protein import channel. At the RNA level, the three ribosomal RNAs are encoded by the mitochondrial genome. In contrast, the transfer RNA (tRNA) population encoded by the plant mitochondrial genome is not sufficient to decode the 61 sense codons of the universal genetic code used by plant mitochondria. It is now well established that to compensate for the lack of tRNAs, several nuclear-encoded tRNAs used by the cytosolic translation machi-nery are also found in the mitochondrion. In this review, evolutionary aspects and functions of imported tRNAs are presented. Then, the basic questions on the tRNA mitochondrial import selectivity, regulation, targeting, and translocation in plants are discussed and compared to what has been discovered in tRNA mitochondrial import in evolutionary divergent organisms. [ABSTRACT FROM AUTHOR]