The present invention is directed to the cells, compositions and methods for the production of recombinant protein, wherein an f-met group on the 5′-terminus is enzymatically removed. In particular, the invention is directed to a production process for obtaining high levels of soluble recombinant CRM197 protein from E. coli. Cells preferably contain one or more mutations of disulfide reductase genes, so that disulfide reductase activity is reduced. The invention also relates to purification method for CRM197 as well as characterization of properly folded CRM197 protein.
