Expression of the Yersinia enterocolitica O:3 LPS O-antigen and outer core gene clusters is RfaH-dependent

The antiterminator RfaH is required for the expression of LPS, capsule, haemolysin, exotoxin, haemin uptake receptor and F pilus. As these structures are critical for bacterial virulence, loss of RfaH usually leads to attenuation. Here, we inactivated the rfaH gene of Yersinia enterocolitica O:3 to study its role in this enteropathogen. RNA sequencing of the WT and ΔrfaH strain transcriptomes revealed that RfaH acted as a highly specific regulator that enhanced the transcription of the operons involved in biosynthesis of LPS O-antigen and outer core (OC), but did not affect the expression of enterobacterial common antigen. Interestingly, the transcriptome of the ΔrfaH strain was very similar to that of an O-antigen-negative mutant. This indicated that some of the changes seen in the ΔrfaH strain, such as the genes involved in outer membrane homeostasis or in the stress-response-associated Cpx pathway, were actually due to indirect responses via the loss of O-antigen. The decreased amount of LPS on the ΔrfaH strain cell surface resulted in an attenuated stress response, and lower resistance to compounds such as SDS and polymyxin B. However, the ΔrfaH strain was significantly more resistant to complement-mediated killing by normal human serum. Taken together, our results revealed a novel role of RfaH acting as a highly specific regulator of O-antigen and OC of LPS in Y. enterocolitica O:3. It may be speculated that RfaH might have an in vivo role in controlling tissue-specific expression of bacterial surface oligo/polysaccharides.