Nodulin-24 gene of soybean codes for a peptide of the peribacteroid membrane and was generated by tandem duplication of a sequence resembling an insertion element

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Abstract
A nodulin gene coding for a polypeptide with an apparent MW of 24,000 (nodulin-24) was isolated from soybean (G. max). DNA sequence analysis of this gene revealed that its coding capacity is for a polypeptide of only 15,000 (MW) and is interrupted by 4 introns. The 3 middle exons and their flanking segments apparently were generated by duplications of a unit resembling an insertion sequence. This unit is bounded by a 12-base-pair inverted repeat and encompasses the 54-base-pair exon corresponding to each of 3 central hydrophobic domains of the protein, nodulin-24. The resulting repeated hydrophobic structure of this protein may be responsible for an apparent increase in MW from 15,100 to 24,000. In vitro translation and immunological studies suggest that nodulin-24 is a precursor and is processed cotranslationally into a 20,000 (MW) polypeptide. This polypeptide is a component of the membrane envelope enclosing the bacteroids (peribacteroid membrane) synthesized during symbiosis with Rhizobium. The low degree (< 6%) of sequence divergence among the repeated units suggests that this gene has been generated recently during the evolution of symbiotic N fixation in soybean.