Ferredoxin component of the biphenyl dioxygenase system that catalyzes the stereospecific dihydroxylation of the aromatic ring of biphenyl, yielding a dihydrodiol compound. Is likely involved in biphenyl degradation that allows growth of Rhodococcus sp. strain RHA1 on biphenyl as the sole source of carbon and energy. The dioxygenase system can also use naphtalene and 4-chlorobiphenyl (4-CB) as substrates, as well as some polychlorinated biphenyls (PCB) such as 2,2'-dichlorobiphenyl, 2,3-dichlorobiphenyl and 2,5,2'-trichlorobiphenyl. It exhibits weak activity toward dibenzofuran and dibenzo-p-dioxin. Electrons are transferred from NADH to the [2Fe-2S] cluster in BphA1 via FAD of BphA4 and [2Fe-2S] cluster of BphA3. Binds 1 [2Fe-2S] cluster per subunit. Xenobiotic degradation; biphenyl degradation. The multicomponent biphenyl dioxygenase system is composed of a ferredoxin reductase (BphA4), a ferredoxin (BphA3), and a terminal oxygenase (BphA1A2). Transcription is up-regulated by aromatic compounds including biphenyl, ethylbenzene, benzene, toluene, xylene, cumene, cymene, and chlorinated benzenes. Is under the control of the BphST two-component regulatory system. Belongs to the bacterial ring-hydroxylating dioxygenase ferredoxin component family. Extended N-terminus.