Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of leg... more Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of legume nodules, a location that coincides with the site of a barrier to O 2 diffusion. The extracellular glycoprotein that binds the monoclonal antibody MAC236 has also been localized to this site. Thus, it has been proposed that these proteins function in the regulation of nodule permeability to O 2 diffusion. It would then be expected that the levels of ENOD2 mRNA/protein and MAC236 antigen would differ in nodules with different permeabilities to O 2 . We examined the expression of ENOD2 and other nodule-expressed genes in Rhizobium melilotiinduced alfalfa nodules grown under 8, 20, or 50% O 2 . Although there was a change in the amount of MAC236 glycoprotein, the levels of ENOD2 mRNA and protein did not differ significantly among nodules grown at the different [O 2 ], suggesting that neither ENOD2 transcription nor synthesis is involved in the long-term regulation of nodule permeability. Moreover, although nodules from all treatments reduced their permeability to O 2 as the partial pressure of O 2 (pO 2 ) was increased to 100%, the levels of extractable ENOD2 and MAC236 proteins did not differ from those measured at the growth pO 2 , further suggesting that if these proteins are involved in a short-term regulation of the diffusion barrier, they must be involved in a way that does not require increased transcription or protein synthesis. ; fax 1-310 -206 -5413.
An alfalfa genomic clone and three cDNA clones for ENOD40 (MsENOD40) have been isolated and chara... more An alfalfa genomic clone and three cDNA clones for ENOD40 (MsENOD40) have been isolated and characterized. At the nucleotide level, the MsENOD40 clones exhibit ca. 79% identity to a soybean (GmENOD40) cDNA clone. The alfalfa cDNA clones lack an AUG translational start codon and potentially encode a polypeptide no longer than 46 amino acids. There is only 39% homology between the putative polypeptides of GmENOD40 and MsENOD40, suggesting that these two proteins may have different functions. However, MsENOD40 transcripts showed a pattern of localization similar to that of GmENOD40 transcripts in that mRNAs were detected by in situ hybridization in the pericycle of inoculated roots, in the nodule primordium, and in stem cells adjacent to the secondary phloem, i.e., the cells of the procambium. In addition, we detected MsENOD40 transcripts in other meristematic cells of the plant, including the nodule meristem, pre-emergent lateral root tips, and the margins of young leaf primordia. The location of MsENOD40 transcripts in dividing cells suggests that the ENOD40 gene product may play a role in mitosis or in associated processes, such as protein synthesis. However, because transcripts were associated with monosomes rather than polysomes, it is likely that MsENOD40 RNA is not translated into protein. Therefore, the RNA itself may have a function in developmental processes.
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