A nodule-specific gene family from Alnus glutinosa encodes glycine- and histidine-rich proteins expressed in the early stages of actinorhizal nodule development.

Two cDNAs representing different members (agNt84 and ag164) of a gene family encoding glycine- and histidine-rich 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 glycine-rich 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 maltose-binding 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.