ABSTRACTExpression of the hypermucoviscosity (HMV) phenotype and capsular polysaccharide (CPS) biosynthesis inKlebsiella pneumoniaewere reported to be encoded by genes located in the chromosomalrmplocus. However, the functions of thermplocus in the virulence plasmid remained unclear, and most of thermploci in clinicalK. pneumoniaeare plasmid carried. In this study, we investigated the functional characteristics of plasmid-bornermphomologues in clinical hypervirulentK. pneumoniae(hvKP) strains by cloning and introducing such gene homologues intoK. pneumoniaestrains of different capsule types, followed by the evaluation of phenotypic changes in these strains. Acquisition of the plasmid-borne prmpADCand prmpA2D2loci were found to result in an increase in mucoviscosity and CPS production in K1 and K2K. pneumoniae, while only the prmpA2D2locus contributed to phenotypic changes in the ST11/KL64 strain. Consistently, bothrmpDandrmpD2increased HMV in K1 and K2K. pneumoniae, while onlyrmpD2contributed to HMV in the ST11/KL64 strain;rmpCcontributed to CPS overproduction in K1 and K2 strains but not in the ST11/KL64 strain. Furthermore, we proposed a logistic molecular basis of the HMV phenotype ofK. pneumoniaeon which prmpD2-mediated HMV is attributed to the increase of cell-free CPS production. Our data confirm that thermphomologues carried by the virulence plasmid play a key role in virulence expression inK. pneumoniae, but the phenotype is highly dependent on the genetic background of the host strain and explained why most of the clinical ST11 strains carry only the prmpA2D2locus.IMPORTANCEKlebsiella pneumoniaehas become the most frequently isolated bacterial pathogen in hospital settings, with a very high mortality rate worldwide. Factors contributing to the virulence ofK. pneumoniaeare the overproduction of capsular polysaccharide (CPS) as well as the hypermucoviscosity (HMV) phenotype. These two phenotypes were reported to be regulated byrmpA/A2homologues, which are often carried by virulence plasmids. Here, we determined the functional role of two plasmid-bornermpAin mediating expression of the HMV phenotype and CPS production inK. pneumoniae. Different capsule types exhibited differences in the expression of HMV and CPS production although they harbored an identical plasmid-bornermpAorrmpA2locus, indicating that these virulence-related phenotypes are strongly related to the genetic background of the host strains. Our study provides a novel understanding of the regulation of virulence-related phenotypes and clinical management ofK. pneumoniaeinfections.