Genes Essential for Amber Disease in Grass Grubs Are Located on the Large Plasmid Found in Serratia entomophila and Serratia proteamaculans.

The bacteria Serratia entomophila and S. proteamaculans cause amber disease in the grass grub, Costelytra zealandica (Coleoptera: Scarabaeidae), an important pasture pest in New Zealand. Disease symptoms include rapid cessation of feeding and amber coloration of larvae. A 105-kb plasmid (designated pADAP) has consistently been found only in pathogenic isolates of both species. Investigations into the involvement of pADAP in amber disease have been hindered by the lack of both a selectable marker on the plasmid and a reliable transposon delivery system. Kanamycin-resistant transposon insertions into three cloned HindIII fragments (9.5, 9.6, and 10.6 kb) were isolated and introduced into pADAP by shuttle mutagenesis. Inserts into the 9.5-and 9.6-kb HindIII fragments on pADAP did not alter disease-causing ability. When plasmids with inserts into the 9.6-kb region were conjugated into plasmid-minus, nonpathogenic isolates of S. entomophila and S. proteamaculans, all of them became pathogenic. Transposon insertions into two regions of the 10.6-kb HindIII fragment continued to cause cessation of feeding but failed to produce amber coloration. Further analysis of a mutant from each amber-minus region (pADK-10 and pADK-13) demonstrated that the antifeeding effect was produced only at dosages higher than that of the wild-type strain. Complementation with the wild-type HindIII fragment restored full-blown disease properties for pADK-13, but not for pADK-10.