Crosstalk between the Type VI Secretion System and the Expression of Class IV Flagellar Genes in the Pseudomonas fluorescens MFE01 Strain

Abstract

Type VI secretion systems (T6SSs) are contractile bacterial multiprotein nanomachines that enable the injection of toxic effectors into prey cells. The <i>Pseudomonas fluorescens</i> MFE01 strain has T6SS antibacterial activity and can immobilise competitive bacteria through the T6SS. Hcp1 (hemolysin co-regulated protein 1), a constituent of the T6SS inner tube, is involved in such prey cell inhibition of motility. Paradoxically, disruption of the <i>hcp1</i> or T6SS contractile tail <i>tssC</i> genes results in the loss of the mucoid and motile phenotypes in MFE01. Here, we focused on the relationship between T6SS and flagella-associated motility. Electron microscopy revealed the absence of flagellar filaments for MFE01Δ<i>hcp1</i> and MFE01Δ<i>tssC</i> mutants. Transcriptomic analysis showed a reduction in the transcription of class IV flagellar genes in these T6SS mutants. However, transcription of <i>fliA</i>, the gene encoding the class IV flagellar sigma factor, was unaffected. Over-expression of <i>fliA</i> restored the motile and mucoid phenotypes in both MFE01Δ<i>hcp1</i>+<i>fliA</i>, and MFE01Δ<i>tssC</i>+<i>fliA</i> and a <i>fliA</i> mutant displayed the same phenotypes as MFE01Δ<i>hcp1</i> and MFE01Δ<i>tssC</i>. Moreover, the FliA anti-sigma factor FlgM was not secreted in the T6SS mutants, and <i>flgM</i> over-expression reduced both motility and mucoidy. This study provides arguments to unravel the crosstalk between T6SS and motility.