LcrQ Coordinates with the YopD-LcrH Complex To Repress Expression and Control Type III Secretion by Yersinia pseudotuberculosis.

Human-pathogenic species employ a plasmid-encoded type III secretion system (T3SS) to negate immune cell function during infection. A critical element in this process is the coordinated regulation of T3SS gene expression, which involves both transcriptional and posttranscriptional mechanisms. LcrQ is one of the earliest identified negative regulators of T3SS, but its regulatory mechanism is still unclear. In a previous study, we showed that LcrQ antagonizes the activation role played by the master transcriptional regulator LcrF. In this study, we confirm that LcrQ directly interacts with LcrH, the chaperone of YopD, to facilitate the negative regulatory role of the YopD-LcrH complex in repressing expression at the posttranscriptional level. Negative regulation is strictly dependent on the YopD-LcrH complex, more so than on LcrQ. The YopD-LcrH complex helps to retain cytoplasmic levels of LcrQ to facilitate the negative regulatory effect. Interestingly, RNase E and its associated protein RhlB participate in this negative regulatory loop through a direct interaction with LcrH and LcrQ. Hence, we present a negative regulatory loop that physically connects LcrQ to the posttranscriptional regulation of LcrF, and this mechanism incorporates RNase E involved in mRNA decay. All three human-pathogenic species, Y. pestis, Y. enterocolitica, and Y. pseudotuberculosis, employ a plasmid-encoded T3SS to target immunomodulatory effectors into host immune cells. Several plasmid-encoded regulators influence T3SS control, including the master transcriptional activator LcrF, the posttranscriptional repressor YopD, and the unassigned negative regulatory factor LcrQ. Since LcrQ lacks any obvious DNA or RNA binding domains, its regulatory mechanism might be special. In this study, we screened for proteins that directly engaged with LcrQ. We found that LcrQ cooperates with LcrH of the YopD-LcrH complex to aid in the posttranscriptional repression of expression. This negative-control loop also involved the mRNA decay factor RNase E and its associated RhlB protein, which were recruited to the regulatory complex by both LcrQ and LcrH. Hence, we identify interacting components of LcrQ that shed new light on a mechanism inhibiting T3SS production and biogenesis.