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The Signal Peptide of the Medicago truncatula Modular Nodulin MtNOD25 Operates as an Address Label for the Specific Targeting of Proteins to Nitrogen-Fixing Symbiosomes

January 2009 , Volume 22 , Number  1
Pages  63 - 72

Natalija Hohnjec,1,2 Frauke Lenz,1 Vera Fehlberg,1 Martin F. Vieweg,1 Markus C. Baier,1 Bettina Hause,3 and Helge Küster1,2

1Institute for Genome Research and Systems Biology (IGS) and 2International Graduate School in Bioinformatics and Genome Research, Center for Biotechnology (CeBiTec), Bielefeld University, D-33594 Bielefeld, Germany; 3Leibniz Institute of Plant Biochemistry, P.O. Box 110432, D-06018 Halle/Saale, Germany

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Accepted 17 September 2008.

The nodule-specific MtNOD25 gene of the model legume Medicago truncatula encodes a modular nodulin composed of different repetitive modules flanked by distinct N- and C-termini. Although similarities are low with respect to all repetitive modules, both the N-terminal signal peptide (SP) and the C-terminus are highly conserved in modular nodulins from different legumes. On the cellular level, MtNOD25 is only transcribed in the infected cells of root nodules, and this activation is mediated by a 299-bp minimal promoter containing an organ-specific element. By expressing mGFP6 translational fusions in transgenic nodules, we show that MtNOD25 proteins are exclusively translocated to the symbiosomes of infected cells. This specific targeting only requires an N-terminal MtNOD25 SP that is highly conserved across a family of legume-specific symbiosome proteins. Our finding sheds light on one possible mechanism for the delivery of host proteins to the symbiosomes of infected root nodule cells and, in addition, defines a short molecular address label of only 24 amino acids whose N-terminal presence is sufficient to translocate proteins across the peribacteroid membrane.

Additional keywords:alternative splicing, confocal laser-scanning microscopy, mRFP.

© 2009 The American Phytopathological Society