The similar to 150 kDa ChlH subunit of magnesium chelatase from Oryza sativa, Hordeum vulgare and Chlamydomonas reinhardtii have been heterologously expressed in Escherichia colt. The active soluble protein is found as both a multimeric and a monomeric form. The multimeric ChlH appears to be oxidatively damaged but monomer production is favoured in growth conditions that are known to cause an oxidative stress response in E. coli. Inducing an oxidative stress response may be of general utility to improve the quality of proteins expressed in E. coli. The similar responses of ChlH's from the three different species suggest that oligomerization of oxidatively damaged ChlH may have a functional role in the chloroplast, possibly as a signal of oxidative stress or damage. (C) 2014 Elsevier Inc. All rights reserved.