Nodulin-100 of soybean is the subunit of sucrose synthase regulated by the availability of free heme in nodules.

A cDNA sequence encoding a nodule-specific protein, nodulin-100, was identified among the abundant transcripts of poly(A)+ RNA from soybean nodules. Purification of nodulin-100 from the soluble fraction of nodule extract yielded an abundant protein with a subunit of approximately 90 kDa having properties of sucrose synthase (UDP-glucose:D-fructose 2-alpha-D-glucosyltransferase, EC 2.4.1.13). Nodule sucrose synthase of soybean is a tetrameric enzyme. Antibodies raised against this protein cross-reacted with the hybrid-released translation product of nodulin-100 cDNA, suggesting that nodulin-100 is the subunit of this enzyme. This was confirmed by partial DNA sequence analysis which showed 73% sequence homology at the amino acid level with maize sucrose synthase. Nodule sucrose synthase was found to dissociate rapidly into monomers in the presence of heme, suggesting that the availability of free heme may regulate the activity of this enzyme. Sucrose synthase activity increases rapidly during nodule development and declines during senescence. A model is presented which suggests that this enzyme plays a key role in maintaining the carbon economy of the nodules and the free heme may be involved in the flow of carbon to the bacteroids. As the degradation of leghemoglobin occurs during senescence, a concomitant decrease in sucrose synthase activity is observed.