The vector developed in this study had to fulfill the following
requirements: endow the transformants with resistance toward
an antibiotic other than ampicillin, because the wild type C. fre-
undii AD970 appeared to be intrinsically resistant to ampicillin,
and allow efficient expression of the heterologous dhaT gene. To
meet these requirements, pJExpress801 was chosen as the back-
bone vector due to its KanR gene and a medium number of copies
of ori (based on pBR322). However, our preliminary results showed
that the rhamnose-induced promoter, native for this vector, was
not efficient with regard to Citrobacter species. We therefore