Multiple sequence alignment of different needle proteins from several bacteria carrying T3SSs. Ser39 and Ser52 are marked with filled black arrowheads. NCBI accession numbers for the needle proteins: Salmonella enterica (PrgI, CBW18951), Chromobacterium violaceum (OQS28935), Providencia alcalifaciens (EUC99172), Pseudomonas fluorescens (MxiH, AKV07676), Pseudogulbenkiania ferrooxidans (ERD99717), enteropathogenic Escherichia coli (EscF, CAS11480), Bordetella pertussis (BscF, CAC79556), Vibrio parahaemolyticus (PscF, KIT21774), Photorhabdus luminescens (LscF, AAO18031), Yersinia enterocolitica (YscF, AAD16839), Yersinia pestis (YscF, NP_395189). The alignment was carried out using Muscle and prepared for visualization using BioEdit. Residues identical are indicated in a black background whereas similar residues are marked gray if they are present in ≥80% of the sequences.