RELATIONSHIP BETWEEN LOSS OF PIGMENTATION AND DELETION OF THE CHROMOSOMAL IRON-REGULATED IRP2 GENE IN YERSINIA-PESTIS - EVIDENCE FOR SEPARATE BUT RELATED EVENTS

The irp2 gene, coding for a 190-kDa iron-regulated protein (HMWP2), an d the hemin storage locus (hms), which determines Yersinia pestis pigm entation, are each located on a large chromosomal fragment which carri es virulence genes and deletes spontaneously. To determine whether the two loci are located on one unstable fragment or on two different exc isable DNA segments, the pigmentation status and the presence of irp2 in 43 strains of Y. pestis isolated in various parts of the world were examined. Three different types were observed: Pgm+ Irp2+ (39.5%), Pg m- Irp2- (44.2%), and Pgm- Irp2+ (16.3%). No Pgm+ Irp2- strain was fou nd. These three types were also recovered in vitro from the parental s train Saigon 55-12-39 (Pgm+ Irp2+), but again, no Pgm+ Irp2- colony wa s observed. Pgm- Irp2- derivatives were obtained from a single Pgm- Ir p2+ colony, indicating sequential loss of the two traits. The fact tha t the genomic SpeI restriction patterns obtained by pulsed-field gel e lectrophoresis were specific for each of the three variants suggested that distinct large-scale chromosomal rearrangements had occurred in t he Pgm- Irp2+ and Pgm- Irp2- derivatives. The virulence of Pgm- Irp2bacteria in mice was ca. 10(7)-fold lower than that of the Pgm- Irp2strains injected subcutaneously but was not significantly decreased wh en injected intravenously. In contrast, the Pgm- Irp2- microorganisms were markedly less pathogenic (10(6)-fold) than the Pgm+ Irp2+ strains injected intravenously and were 100 times less virulent than the Pgm- Irp2+ strains injected subcutaneously.