Purified recombinant proteins fromSynechocystisPCC6803 were used to show that the magnesium chelatase ChlH subunit stimulates magnesium protoporphyrin methyltransferase (ChlM) activity. Steady-state kinetics demonstrate that ChlH does not significantly alter theKmfor the tetrapyr-role substrate. However, quenched-flow analysis reveals that ChlH dramat-ically accelerates the formation and breakdown of an intermediate in the catalytic cycle of ChlM.