Expression system for clostridium species

A system is used to express clostridial gene constructions in a clostridial host. A mobilizable transfer plasmid is described which permits the direct transfer of the plasmid, and genes carried on it, from E. coli into Clostridium species. A promoter is described for use in clostridial species. Also, a useful host strain is used which is nontoxigenic and which permits high levels of expression of clostridial genes using the clostridial promoter.