IntroductionActinobacillus pleuropneumoniae(A. pleuropneumoniae) is a member ofActinobacillusin family Pasteurellaceae. It is the causative agent of porcine pleuropneumonia, which has caused huge economic losses to pig industry over the world. Cysteine is a precursor of many important biomolecules and defense compounds in the cell. However, molecular mechanisms of cysteine transport inA. pleuropneumoniaeare unclear.MethodsIn this study, gene-deleted mutants were generated and investigated, to reveal the roles of potential cysteine/cystine transport proteins FliY and YdjN ofA. pleuropneumoniae.ResultsOur results indicated that the growth ofA. pleuropneumoniaewas not affected afterfliYorydjNsingle gene deletion, but absence of both FliY and YdjN decreased the growth ability significantly, when cultured in the chemically defined medium (CDM) supplemented with cysteine or cystine as the only sulfur source.A. pleuropneumoniaedouble deletion mutant ΔfliYΔydjNshowed increased sensitivity to oxidative stress. Besides,trans-complementation of YdjN into ΔfliYΔydjNand wild type leads to increased biofilm formation in CDM. However, the virulence of ΔfliYΔydjNwas not attenuated in mice or pigs.DiscussionThese findings suggest thatA. pleuropneumoniaeFliY and YdjN are involved in the cysteine/cystine acquisition, oxidative tolerance, and biofilm formation, but not contribute to the pathogenicity ofA. pleuropneumoniae.