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A Nodule-Specific Gene Family from Alnus glutinosa Encodes Glycine- and Histidine-Rich Proteins Expressed in the Early Stages of Actinorhizal Nodule Development

¹Department of Molecular Biology, Agricultural University, 6703 HA Wageningen, The Netherlands; ²Department of Biology, University of Nebraska at Kearney, Kearney 68849, U.S.A; ³Department of Botany and Center for Legume Research, University of Tennessee, Knoxville 37996, U.S.A.; ⁴Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences, Pushchino, Moscow Region, 142292, Russia

Two cDNAs representing different members (agNt84 and ag164) of a gene family encoding glycine- and histidinerich proteins have been isolated from cDNA libraries from Alnus glutinosa root nodules. Expression of the corresponding genes could only be detected in nodules. With in situ hybridization, the expression in nodules was found to occur in young, infected cells of the prefixation zone (zone 2). The encoded proteins contain putative signal peptides for targeting to the endomembrane system, sharing sequence similarity with signal peptides from plant glycinerich proteins, among them nodulin 24, a nodule-specific protein from soybean. This similarity suggests that, analogous to nodulin-24, proteins encoded by agNt84/ag164 may be located at the interface between the host plant membrane and the matrix surrounding the endosymbiont. The 3′untranslated regions of the cDNAs contain unusual poly(AT)_n stretches that may play a role in the regulation of RNA stability. The protein encoded by agNt84 cDNA was expressed in Escherichia coli as a fusion with maltosebinding protein, and was shown to have the ability to bind to a nickel-chelating resin, indicating that it may function as a metal-binding protein.

Additional keywords: actinorhizal, nitrogen fixation.