Inactivation of the <i>Fusobacterium nucleatum</i> Rnf complex reduces FadA-mediated amyloid formation and tumor development

The Gram-negative anaerobe <i>Fusobacterium nucleatum</i> is an oral oncobacterium that promotes colorectal cancer (CRC) development with the amyloid-forming cell surface adhesin FadA integral to CRC tumorigenesis. We describe here molecular genetic studies uncovering a novel mode of metabolic regulation of FadA-mediated tumor formation by a highly conserved respiratory enzyme known as the Rnf complex. First, we show that genetic disruption of Rnf, via <i>rnfC</i> deletion, significantly reduces the level of <i>fadA</i> transcript, accompanied by a near-complete abolishment of the precursor form of FadA (pFadA), reduced assembly of FadA at the mature cell pole, and severe defects in the osmotic stress-induced formation of FadA amyloids. We show further that the Rnf complex regulates three response regulators (CarR, ArlR, and S1), which modulate the expression of pFadA, without affecting <i>fadA</i> transcript. Consistent with our hypothesis that these response regulators control factors that process FadA, deletion of <i>rnfC</i>, <i>carR</i>, <i>arlR</i>, or <i>s1</i> each impairs expression of the signal peptidase gene <i>lepB</i>, and FadA production is nearly abolished by CRISPR-induced depletion of <i>lepB</i>. Importantly, while <i>rnfC</i> deletion does not affect the ability of the mutant cells to adhere to CRC cells, <i>rnfC</i> deficiency significantly diminishes the fusobacterial invasion of CRC cells and formation of spheroid tumors <i>in vitro</i>. Evidently, the Rnf complex modulates the expression of the FadA adhesin and tumorigenesis through a gene regulatory network consisting of multiple response regulators, each controlling a signal peptidase that is critical for the post-translational processing of FadA and surface assembly of FadA amyloids.IMPORTANCEThe <i><u>R</u>hodobacter</i> <u>n</u>itrogen-<u>f</u>ixation (Rnf) complex of <i>Fusobacterium nucleatum</i> plays an important role in the pathophysiology of this oral pathobiont since genetic disruption of this conserved respiratory enzyme negatively impacts a wide range of metabolic pathways, as well as bacterial virulence in mice. Nonetheless, how Rnf deficiency weakens the virulence potential of <i>F. nucleatum</i> is not well understood. Here, we show that genetic disruption of the Rnf complex reduces surface assembly of adhesin FadA and FadA-mediated amyloid formation, via regulation of signal peptidase LepB by multiple response regulators. As FadA is critical in the carcinogenesis of colorectal cancer (CRC), the ability to invade CRC cells and promote spheroid tumor growth is strongly diminished in an Rnf-deficient mutant. Thus, this work uncovers a molecular linkage between the Rnf complex and LepB-regulated processing of FadA-likely via metabolic signaling-that maintains the virulence potential of this oncobacterium in various cellular niches.