Suppression of a Transketolase Mutation Leads to Only Partial Restoration of Symbiosis in Sinorhizobium meliloti.

Abstract

The interaction between Sinorhizobium meliloti and alfalfa is a well-studied model system for symbiotic establishment between rhizobia and legume plants. Proper utilization of carbon sources has been linked with effective symbiotic establishment in S. meliloti strain Rm1021. Previous work has shown that mutation of the gene tktA, which encodes a transketolase involved in the pentose phosphate pathway, resulted in a strain impaired in many biological functions, including the inability to establish a symbiosis with alfalfa. Work with this strain revealed the appearance of suppressor mutations which could partially revert the symbiotic phenotype associated with a tktA mutation. Characterization of these suppressor strains revealed that carbon phenotypes associated with a mutation in tktA were no longer present and that the production of succinoglycan was partially restored. Central carbon metabolite pools were observed to be different compared to the wildtype and tktA mutant strains. Multiple independent mutations were identified in the gene SMc02340, a Gnt-type negative regulator upon sequencing. RT-PCR suggests that SMc02340 acts as a negative regulator on an operon containing the gene tktB, which becomes upregulated when the suppressor mutation is present or SMc02340 is removed. Microscopic analysis revealed a unique symbiotic phenotype. The tktA mutant strain induced root hair curling but could not colonize the apoplastic space. Collectively the data suggests the upregulation of tktB can partially bypass some blocks associated with a lesion in tktA, including the colonization of the curled root hair, but cannot fully compensate for the loss of tktA.