Isolation of Two New Sinorhizobium meliloti Transcriptional Regulators Required for Nodulation

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In the absence of fixed nitrogen, Sinorhizobium meliloti establishes an effective nitrogen fixing symbiosis with its legume partner, alfalfa (Medicago sativa) through a series of intricate signal exchanges and reciprocal structural changes. The signal exchanges between the two organisms starts with the alfalfa flavonoids and the S. meliloti nodulation factors. This exchange of signals results in the formation of tightly curled alfalfa root hairs. S. meliloti cells elicit the formation of infection threads inside these curled root hairs. The infection threads elongate toward the base of root hairs, penetrating layers of alfalfa root cells, reaching developing nodule primordium, and releasing live bacterial cells into the plant cells inside the nodule primordium. The successful differentiation of the S. meliloti cells may depend on another set of signal exchanges between S. meliloti and alfalfa involving proteins like BacA. The complexity of the S. meliloti-alfalfa interaction raises the possibility that many more signal exchanges between the two organisms remain unknown. Some of the key regulatory genes involved in the signal exchanges between S. meliloti and alfalfa belong to the family of LysR transcriptional regulators. The production of nodulation factor that is part of the signal exchanges that initiate symbiosis, is positively controlled by three different nodD genes: nodD1, nodD2 and nodD3 and the syrM gene. The nod factor production is also negatively regulated by the nolR gene (9). The nolR gene encodes a repressor that negatively regulates the expression of the nodD1 and nodABC operons by binding to the divergent promoter between them. The genomic sequence has been determined for S. meliloti Rm1021 and 90 putative lysR family transcriptional regulators have been identified. Each of 90 previously unknown putative S. meliloti lysR genes was interrupted by insertions of suicide plasmids into the middle of the open reading frames.