Zymograms showing SDS hydrolysis after SDS-PAGE of crude extract of different Y . ruckeri strains. SDS-PAGE of bacterial cell extracts was performed at 15 mA in a cool room for 16 h. Then, gels were incubated at 20°C for 4 h and kept 1 additional hour at 4°C for SDS precipitation. Bands of SDS hydrolysis activity appear as clear zones against an opaque gel. (A) Cell extract from: lane 1, Y. ruckeri (parental strain); lane 2, yraS-; lane 3, yraS+; lane 4, parental strain heated at 100°C for 10 min. (B) Cell extract from: lane 1, Y. ruckeri 956; lane 2, 956yraS+. Molecular mass markers (in kDa) are indicated on the left side of each gel. Photographs were taken on a dark background to contrast the bands. There was a match between the appearance in the SDS-PAGE of the 120 kDa SDS hydrolysis bands and the presence of the yraS gene in the strains.