Soybean nodulin genes: Analysis of cDNA clones reveals several major tissue-specific sequences in nitrogen-fixing root nodules

Abstract

Plant gene products that could play a role in the process of symbiotic N2 fixation in leguminous plants were detected by screening a cDNA [complementary DNA] library prepared from soybean nodule poly(A)+ RNA. About 13% of the 5700 clones screened contained sequences detectable with a root cDNA probe, while about 2100 clones (37% of the library) contained sequences that were detectable only with nodule cDNA. Five unique sequence species, accounting for more than half of the 2100 nodule-specific clones, were identified by cross-hybridization experiments. The most abundant species, represented by 860 clones, encodes the well characterized protein, leghemoglobin (Lb). The other 4 spp., designated NodA, NodB, NodC and NodD, are represented by 350, 55, 61 and 6 clones, respectively. Each of these 4 spp. was encoded by the plant nuclear genome at low copy number. The transcripts corresponding to the nodule-specific clones represented 12-15% (Lb), 6% (NodA) and 0.5-1.1% (NodB, NodC and NodD) mole fraction of nodule polysomal mRNA but could not be detected in root polysomal RNA. Hybrid-selection of nodule mRNA by representative clones and in vitro translation indicated that polypeptides of MW 44,000, 27,000, 24,000 and 100,000-120,000 are encoded by NodA, NodB, NodC and NodD sequences, respectively. These polypeptides reacted with antiserum prepared against total soluble nodule proteins, suggesting that the cloned sequences encode nodule-specific proteins, nodulins.