Bactericidal Type IV Secretion System Homeostasis in Xanthomonas citri

Abstract

11Several Xanthomonas species have a type IV secretion system (T4SS) that injects a cocktail of 12 antibacterial proteins into neighbouring Gram-negative bacteria, often leading to rapid lysis 13 upon cell contact. This capability represents an obvious fitness benefit since it can eliminate 14 competition while the liberated contents of the lysed bacteria could provide an increase in 15 the local availability of nutrients. However, the production of this Mega Dalton-sized T4SS, 16with over a hundred subunits, also imposes a significant metabolic cost. Here we show that 17 the chromosomal virB operon, which encodes the entirety of structural genes of the T4SS in 18 X. citri, is regulated by the global regulator CsrA. Relieving CsrA repression from the virB 19 operon produced a greater number of T4SSs in the cell envelope and an increased efficiency 20 in contact dependent lysis of target cells. However, this was also accompanied by a 21 physiological cost leading to reduced fitness when in co-culture with wild-type X. citri. We 22show that T4SS production is constitutive despite being downregulated by CsrA. Cells 23 subjected to a wide range of rich and poor growth conditions maintain a constant density of 24 T4SSs in the cell envelope and concomitant interbacterial competitiveness. These results 25show that CsrA provides a constant though partial repression on the virB operon, 26 independent of the tested growth conditions, in this way controlling T4SS-related costs while 27 at the same time maintaining X. citri's aggressive posture when confronted by competitors. 28 29 2

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“…This may lead to increased cellular adhesion, similar to what is observed in the xopQ- mutant. Furthermore, the KEGG analysis revealed components of type IV secretion system to be up-regulated in the xopQ- mutant and previous results indicate this secretion system is involved in promoting biofilm formation (Cenens, Andrade et al, 2020, Elhenawy, Hordienko et al, 2021, Seifert, 2017). Notably, components of the type III secretion apparatus were amongst down-regulated pathways in the xopQ- mutant (Appendix Table S5).…”