Part of the oxygenase component of the biphenyl dioxygenase system that catalyzes the stereospecific dihydroxylation of the aromatic ring of biphenyl, yielding a dihydrodiol compound. Is essential for biphenyl degradation and growth of Rhodococcus sp. strain RHA1 on biphenyl as the sole source of carbon and energy. 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. 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. biphenyl + H(+) + NADH + O2 = (2R,3S)-3-phenylcyclohexa-3,5-diene-1,2-diol + NAD(+) Binds 1 [2Fe-2S] cluster per subunit. Binds 1 Fe cation per subunit. Xenobiotic degradation; biphenyl degradation; 2-hydroxy-2,4-pentadienoate and benzoate from biphenyl: step 1/4. Heterohexamer consisting of three BphA1 subunits and three BphA2 subunits. 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. Cells lacking this gene lose their ability to grow on biphenyl. Belongs to the bacterial ring-hydroxylating dioxygenase alpha subunit family.