rRNA operon ploidy correlates to Bradyrizobium cell physiology.

LEILA BARTROLÍ; CAROLA CAMPANELLI; SOLER BISTUE, ALFONSO; IAN MEDICI; COMERCI, DIEGO J.; ALEXIS ROMANI; MONGIARDINI, ELIAS

Virtual

Congreso; Congreso SAIB SAMiGe 2020; 2020

SAIB y SAMiGe

Growth rate (GR) is a key parameter that reflects bacterial physiology and fitness. The genetic factors encoding it are still unknown. Genome comparisons showed that the number of rRNA operons (rrn) and their proximity to the replication origin closely correlate to GR. Bacteria bear from 1 to 16 rrn. Those having a higher copy number grow faster. Bradyrhizobia are economically important group since they interact symbiotically with soy, enabling the biological fixation of nitrogen by the plant increasing crop productivity. Bradyrhizobiaceae shows a particularly slow GR which makes its study difficult and impacts on its biotechnological utility. Indeed, cell phsysiology of this clade is not well characterized. Close examination of complete genomes within this group shows high homology among different isolates and that this clade bears only 1 or 2 rrn. We hypothesized that species with 2 rrn should grow faster. To test this, we used as models completely sequenced B. diazoeficiens and B. japonicum strains bearing 1 and 2 rrns respectively. Growth curves of these strains showed that B. japonicum displayed a shorter generation time and lag phase. Time-lapse microscopy showed that Bradyrizobia display an extreme asymmetric division and non-canonical coordination of cell size and cell cycle. Nevertheless, as in other model systems and in line with our previous results, B. japonicum displayed higher cell length than B. diazoeficiens. Also, competition experiments in broth showed that B. japonicum displays higher fitness than strains with a single rrn. In sum, we collected a body of evidence showing that rrn copy-number impact Bradyrhizobium physiology and fitness. Strains with 2 rrns grow faster, have a shorter lag phase and a greater cell size than those bearing a single operon. They also outcompete cell with a single rrn in pairwise competitions. We are currently modifying rrn ploidy within the same genetic background to be able to demonstrate causation. In further work we aim at introducimg additional rrn copies to try to artificially increase the GR of these extremely slow-growing microorganisms.