BchI, belonging to the AAA(+)-protein family, forms the enzyme magnesium chelatase together with BchD and BchH. This enzyme catalyses the insertion of Mg2+ into protoporphyrin IX upon ATP hydrolysis. Previous studies have indicated that BchI forms ATP-dependent complexes and it is a member of the AAA(+)-protein family (ATPases associated with various cellular activities) and it was suggested based on structural homology that the Bchl formed hexameric complexes. AAA(+)-proteins are Mg2+-dependent ATPases that normally form oligomeric ring complexes in the presence of ATP. Single particle analysis of fully formed ring complexes of Bchl observed by negative staining EM indicate that the BchI has strong 6- and 2-fold rotational symmetries and a weaker 4-fold rotational symmetry which are reminiscent of DNA helicase. A 2D average of the fully formed BchI ATP ring complex is presented here from images of the complex obtained from negative staining EM. Other complexes are also observed in the EM micrographs and the class averages of these are indicative of the fragility and dynamic nature of the BchI complex which has been reported and they are suggestive of partially circular complexes with six or less protomers per particle. The resolution of the average circular complex is estimated at similar to30 A and it is similar in shape and size to an atomic resolution hexameric model of BchI rendered at similar to30 Angstrom. (C) 2003 Elsevier Inc. All rights reserved.