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Participation of Leaky Ribosome Scanning in Protein Dual Targeting by Alternative Translation Initiation in Higher Plants^[W],[OA]

^a Center for Plant Science Innovation, University of Nebraska, Lincoln, Nebraska 68588-0660
^b Center for Biotechnology, University of Nebraska, Lincoln, Nebraska 68588-0660

¹ Address correspondence to smackenzie2{at}unl.edu.

Postendosymbiotic evolution has given rise to proteins that are multiply targeted within the cell. Various mechanisms have been identified to permit the expression of proteins encoding distinct N termini from a single gene. One mechanism involves alternative translation initiation (aTI). We previously showed evidence of aTI activity within the Arabidopsis thaliana organellar DNA polymerase gene POL2. Translation initiates at four distinct sites within this gene, two non-AUG, to produce distinct plastid and mitochondrially targeted forms of the protein. To understand the regulation of aTI in higher plants, we used Pol2 as a model to investigate both cis- and trans-acting features of the process. Here, we show that aTI in Pol2 and other plant genes involves ribosome scanning dependent on sequence context at the multiple initiation sites to condition specific binding of at least one trans-acting factor essential for site recognition. Multiple active translation initiation sites appear to operate in several plant genes, often to expand protein targeting. In plants, where the mitochondrion and plastid must share a considerable portion of their proteomes and coordinate their functions, leaky ribosome scanning behavior provides adaptive advantage in the evolution of protein dual targeting and translational regulation.

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