The A-type flavoproteins (ATF) are modular proteins involved in multi-component electron transfer pathways, having oxygen reductase activity. They are complex flavoproteins containing two distinct structural domains, one having an FMN in a flavodoxin-like fold and the other a binuclear iron centre within a metallo-beta-lactamase-like fold. Here, we report the purification and characterisation of a recombinant ATF from the cyanobacterium Synechoystis sp. PCC 6803, which has the unique feature of comprising an additional third domain with similarities towards flavin:NAD(P)H reductases. The latter was expressed independently as a truncated protein form and found to be capable of receiving electrons from NADH as well as to indiscriminately bind either one FAD or one FMN with equivalent affinities. Further kinetic studies have shown that the intact ATF is an NADH:oxygen oxidoreductase, with the catalytic ability to fully reduce oxygen to water. Thus, this constitutes an example on how structural modules found within partner proteins from an electron transfer pathway can be combined in a single polypeptide chain achieving identical catalytic activities. (C) 2002 Elsevier Science (USA). All rights reserved.