Figure 4. Identification of std operon products involved in SPI-1 repression.

A. ß-galactosidase activities of invF::lac and sipB::lac fusions in a strain containing an intact std operon (control) and in a isogenic strain lacking the entire std operon (Δstd). Black and white histograms represent ß-galactosidase activities in Dam⁺ and Dam⁻ backgrounds, respectively. Asterisks above columns indicate that the differences observed are statistically significant (P<0.005). B. Regulation of an invF::lac fusion by Dam methylation in strains carrying in-frame deletions in individual std genes. Histograms represent ß-galactosidase activities in a Dam⁺ background (black) and in a Dam⁻ background (white). Asterisks above columns indicate that the differences observed are statistically significant (P<0.005).