We determined the complete nucleotide sequence of a 2.1-kb HindIII-EcoRI fragment that was cloned from a resident large plasmid of Klebsiella pneumoniae Chedid, a highly virulent and mucoviscous strain of the O1:K2 serotype. This fragment encoded an ability to enhance K2 capsular polysaccharide synthesis in K. pneumoniae, and a 636-bp open reading frame (rmpA2) was found. The 411-bp rmpA reported to be involved in the virulence and mucoid phenotypes of K. pneumoniae by Nassif et al. (Mol. Microbiol. 3:1349-1359, 1989) was a part of rmpA2. Eighty percent homology in nucleotide sequence was found between rmpA2 and rmpA in the corresponding regions. The central domain of the deduced amino acid sequence of RmpA2 showed considerable homology to the central domains of NtrC of K. pneumoniae and Escherichia coli, to which the sigma factor of RNA polymerase binds. The C-terminal domain of RmpA2 also demonstrated considerable homology with the putative helix-turn-helix motifs of LuxR of Vibrio fischeri and FixJ of Rhizobium meliloti. Moreover, RmpA2 also showed some homology in its N- and C-terminal regions to those of RcsA, a transcriptional activator for colanic acid synthesis in E. coli. On the other hand, a sequence upstream of rmpA2 was found to be highly homologous to insertion sequence 3 of members of the family Enterobacteriaceae. Southern hybridization analysis suggested that rmpA2 exists on the large plasmids of all mucoviscous virulent K2 strains but not on those of the slightly mucoviscous avirulent strains. Freeze substitution electron microscopy and fluorescent-antibody staining with anti-K2 serum revealed that K. pneumoniae Chedid has a dense and thick capsule (180 nm) with dense extracapsular substance, whereas K. pneumoniae K2-215, one of the slightly mucoviscous and avirulent strains, has a capsule which is looser and thinner (120 nm) than that of strain Chedid and no extracapsular substance. Introduction of rmpA2 into K2-215 as well as reference strains K. pneumoniae K9 and K72 resulted in a change of the colony phenotype to highly mucoviscous through abundant production of extracapsular substance which reacted with anti-K2, -K9, or -K72, respectively, as did their parental strains. From these results, it is suggested that RmpA2 belongs to the family of transcriptional regulators and confers a highly mucoviscous phenotype on cells of various serotypes of K. pneumoniae by enhancing extracapsular polysaccharide synthesis.