Required for ESX-1 function (PubMed:16135231, PubMed:22155774). Required for the maintenance of adequate cellular levels of both EspA and EspC (PubMed:22155774). Facilitates EsxA secretion (PubMed:22155774). Interacts with EccE1. Export does not exclusively require ESX-1. May require one (or more) of the other ESX / type VII secretion systems (T7SS). Transcriptionally activated by EspR (PubMed:18685700, PubMed:22389481, PubMed:25536998). Repressed by the MprB/MprA two-component system, by direct regulation and via EspR (PubMed:23104803, PubMed:25536998). Up-regulated by the PhoP/PhoR two-component system, via EspR (PubMed:16573683, PubMed:25536998). EspD deficiency results in decreased cellular levels of EspA and EspC, but does not affect their transcription.