The nucleoid protein HU positively regulates the expression of type VI secretion systems in .

Unlabelled: is an emerging pathogen isolated in healthcare-associated infections. A major virulence factor of this bacterium is the type VI secretion system (T6SS). The genome of harbors two T6SS gene clusters (T6SS-1 and T6SS-2), and the functional characterization of both systems showed that these two T6SSs are not expressed under the same conditions. Here, we report that the major histone-like protein HU positively regulates the expression of both T6SSs and, therefore, the function that each T6SS exerts in . Single deletions of the genes encoding the HU subunits ( and ) decreased mRNA levels of both T6SS. In contrast, the double mutant dramatically affected the T6SS expression, diminishing its transcription. The direct binding of HU to the promoter regions of T6SS-1 and T6SS-2 was confirmed by electrophoretic mobility shift assay. In addition, single and double mutations in the genes affected the ability of inter-bacterial killing, biofilm formation, adherence to epithelial cells, and intestinal colonization, but these phenotypes were restored when such mutants were -complemented. Our data broaden our understanding of the regulation of HU-mediated T6SS in these pathogenic bacteria.

Importance: T6SS is a nanomachine that functions as a weapon of bacterial destruction crucial for successful colonization in a specific niche. expresses two T6SSs required for bacterial competition, adherence, biofilm formation, and intestinal colonization. Expression of T6SS genes in pathogenic bacteria is controlled by multiple regulatory systems, including two-component systems, global regulators, and nucleoid proteins. Here, we reported that the HU nucleoid protein directly activates both T6SSs in , affecting the T6SS-related phenotypes. Our data describe HU as a new regulator involved in the transcriptional regulation of T6SS and its impact on pathogenesis.