Matthew S. Francis

YscX was discovered as an essential part of the Yersinia type III secretion system about 20 years ago. It is required for substrate secretion and is exported itself. Despite this central role, its precise function and mode of action remain unknown. In order to address this knowledge gap, this present study refocused attention on YscX to build on the recent advances in the understanding of YscX function. Our experiments identified an N‐terminal secretion domain in YscX promoting its secretion, with the first five codons constituting a minimal signal capable of promoting secretion of the signal less β‐lactamase reporter. Replacing the extreme YscX N‐terminus with known secretion signals of other Ysc‐Yop substrates revealed that the YscX N‐terminal segment contains non‐redundant information needed for YscX function. Further, both in cis deletion of the YscX N‐terminus in the virulence plasmid and ectopic expression of epitope‐tagged YscX variants again lead to stable YscX production but not type III secretion of Yop effector proteins. Mislocalisation of the needle components, SctI and SctF, accompanied this general defect in Yops secretion. Hence, a coupling exists between YscX secretion permissiveness and the assembly of an operational secretion system.