Iron-sulfur cluster subunit of the adenylylsulfate reductase which catalyzes reversibly the reduction of adenosine 5'-phosphosulfate (APS) to sulfite and AMP during dissimilatory sulfate reduction. The iron-sulfur cluster 2 is thought to accept electrons from a still unknown electron donor and transfer electrons to the iron-sulfur cluster 1 of this protein and then onto the FAD of AprA. Binds 2 [4Fe-4S] clusters. Optimum pH is 7.4 for the adenylylsulfate reductase. E(0) is about 0 mV for 4Fe-4S cluster 1 and is estimated to be lower than -400 mV for a fully reduced 4Fe-4S cluster 2. Heterodimer composed of AprA and AprB (PubMed:19820092, PubMed:2158885). The heterodimers can dimerize to form heterotetramers (PubMed:2158885). Consists of three sections from the N- to the C-terminus: [4Fe-4S] cluster-binding domain, beta-sheet and C-terminal tail. The beta-sheet and the C-terminal tail interact with AprA to stabilize the AprA/AprB heterodimer.