Novel, highly expressed late nodulin gene (LjNOD16) from Lotus japonicus

We have isolated a Lotus japonicus cDNA corresponding to a highly abundant, late nodule-specific RNA species that encodes a polypeptide with a predicted molecular mass of 15.6 kD. The protein and its corresponding gene were designated Nlj16 and LjNOD16, respectively. LjNOD16 was found to be expressed only in the infected cells of L. japonicus nodules. Related DNA sequences could be identified in the genomes of both Glycine max and Medicago sativa. In the latter, a homologous mRNA species was detected in the nodules. Unlike LjNOD16, its alfalfa homologs appear to represent low-abundance mRNA species. However, the proteins corresponding to the LjNOD16 and its alfalfa homolog could be detected at similar levels in nodules but not in roots of both legume species. The predicted amino acid sequence analysis of nodulin Nlj16 revealed the presence of a long alpha-helical region and a positively charged C terminus. The former domain has a very high propensity to form a coiled-coil type structure, indicating that nodulin Nlj16 may interact with an as-yet-unidentified protein target(s) in the nodule-infected cells. Homology searches revealed no significant similarities to any known sequences in the databases, with the exception of two related, anonymous Arabidopsis expressed sequence tags.