Regulation of hsvG, a host-specific virulence gene from Erwinia herbicola pv. gypsophilae

Erwinia herbicola pv. gypsophilae (Ehg) incites galls only on gypsophila whereas E. herbicola pv. betae (Ehb) elicits galls on both beet and gypsophila, hsvG is a positive-acting pathogenicity gene that determines host specificity of both pathovars on gypsophila. By employing the ice nucleation reporter gene inaZ it was found that the transcriptional activity of hsvG in Ehg cells grown in gypsophila stems was induced by 3 orders of magnitude after 24 h. A similar induction rate was also detected in carnation or bean stems. hsvG inaZ activity on bean leaf surfaces was 2 orders lower than in bean stems. Over-expression of HrpL or HrpS stimulated the transcriptional activity of hsvG in LB or minimal media by approx. 2 orders but hardly affected the activity in gypsophila stems. Inactivation of either hrpL or hrpS caused a reduction of 3 4.5 orders in hsvG inaZ activity in bacteria grown on LB or minimal A media and in gypsophila stems. HsvG protein was detected by Western blotting in Ehg cells only during overproduction of HrpL or HrpS. Inactivation of hsvG reduced the hrp-dependent multiplication of Ehg in gypsophila stems by 1 order. Hrp-dependent endophytic growth was detected in gypsophila stems but not in carnation or bean stems.