degP1 regulation in Sinorhizobium meliloti: Is there a crosstalk between ActJK and CpxAR two-component systems?

VACCA, CAROLINA; PEREZ, AGUSTINA; BELHART, KEILA; DRAGHI, WALTER OMAR; LAGARES, ANTONIO; NIEHAUS, KARSTEN; DEL PAPA, MARÍA FLORENCIA

Congreso; 3er Congreso ISME-Latr; 2023

The nitrogen-fixing symbiosis between Sinorhizobium meliloti and Medicago sativa (alfalfa) is affected in acid stressing soils, a worldwide distributed condition that is detrimental for the correct plant nutrition. Thus, to explore the responses of S. meliloti to acidity, we used an interdisciplinary approach trying to link genes and proteins to the physiology of the rhizobial responses under low pH as an abiotic factor that modulates microbial niches .ActJK is a two-component system of S. meliloti which plays an essential role in the survival of bacteria at low pH, and also in their symbiosis with alfalfa. This acid-activated regulatory system provides insights into the evolutionary strategies that may assist bacterial adaptation to acidic environments. To advance on the description of the ActJ regulon, the proteomes of S. meliloti 2011 wild type and its ΔactJ isogenic mutant―both cultivated at neutral or acid pH―were compared to identify proteins under the ActJ control. Furthermore, to evaluate specific transcriptional activities, different promoters’ activities were analyzed by using a set of selected promoter fusions to eGFP.Results showed that under low pH there is an increase in the DegP1 protease levels suggesting a higher rate of protein degradation and turnover, probably due to increased protein damage. In addition, at low pH an increased transcription of degP1 and actJ in an ActJ-dependent manner occurred. It has been reported that in E. coli (and in other bacterial genera) DegP expression is regulated by the CpxRA two component system. To investigate if that was also the case in S. meliloti, the possible involvement of CpxRA and the degP1 expression under acid stress (at various pHs) were evaluated. To do that, we constructed and characterized in-frame deletion mutants of cpxR and cpxA and demonstrated that, under acidic conditions, ActJ is a regulator which controls degP1 transcription ruling out the existence of crosstalk between ActK and CpxR.The results presented here provide novel information and insight into the regulatory molecular pathways that operate when S. meliloti is challenged by low pH, and provide new targets for selection/modification when attempting a rational search for rhizobial strains with improved survival and symbiotic performance in moderately acid soils.