Coupling Secretion with Transcription

Abstract

Bacterial virulence factor secretion systems, which contribute to pathogenicity, are assembled when bacteria encounter host cells. Mycobacterium tuberculosis , which causes tuberculosis, produces a type VII secretion system (also called the ESX-1 protein secretion system). In a screen for ESX-1 secretion mutants, Raghavan et al . identified Rv3849 , which encodes a secreted protein that they named EspR for ESX-1 secreted protein regulator. In wild-type bacteria, EspR was secreted by the secretion system. However, mutants defective in EspR function exhibited not only lack of EspR secretion but also diminished secretion of other factors, such as ESAT-6. Thus, EspR was not only a substrate of the secretion system but also required for proper function of the ESX-1 system. Secondary structure modeling suggested that EspR had a helix-turn-helix DNA binding domain and in the Rv3849 − cells three operons were activated and two operons were less active, including one Rv3616c - Rv3612c that encodes proteins that are necessary for the ESX-1 system. After phagocytosis by macrophages, Rv3849 − cells exhibited decreased induction of the Rv3616c - Rv3612c operon. Expression of M. tuberculosis EspR in a nonpathogenic relative that also expressed a Rv3616c promoter reporter showed that EspR stimulated transcription, and electrophoretic mobility shift assays confirmed that EspR bound the Rv3616c promoter. Forced expression of Rv3616c - Rv3612c using a constitutive promoter restored ESX-1 function in EspR-deficient bacteria, suggesting that the transcriptional activity is the relevant mechanism by which EspR contributes to ESX-1 secretion. Addition of an epitope tag to EspR blocked its secretion without compromising its transcriptional activity, and expression of the tagged EspR resulted in enhanced repression of EspR-inhibited genes and enhanced expression of EspR-activated genes. Thus, secretion of EspR appears to serve as a homeostatic mechanism to couple secretion with transcription of the genes encoding secretory system components. S. Raghavan, P. Manzanillo, K. Chan, C. Dovey, J. S. Cox, Secreted transcription factor controls Mycobacterium tuberculosis virulence. Nature 454 , 717-721 (2008). [PubMed]