In vitro characterization of transcriptional-fusions to virulence genes As a first application of the two-colour flow cytometric technique , the in-vivo activities of three promoters that drive the expression of genes with differential roles in virulence were studied : PsicA , which drives the expression of the operon sicA sipBCDA iacP within SPI1 ( Darwin and Miller , 2000 ) , P ( Valdivia and Falkow , 1997 ; called ssaH PssaG by Hensel et al. , 1998 ) , which drives the expression of the operon ssaHIJKLMV within SPI2 ( Cirillo et al. , 1998 ) , and PpagC , which drives the PhoP-activated expression of pagC ( Gunn et al. , 1995 ) .