Regulatory subunit of casein kinase II/CK2 (By similarity). As part of the kinase complex regulates the basal catalytic activity of the alpha subunit a constitutively active serine/threonine-protein kinase that phosphorylates a large number of substrates containing acidic residues C-terminal to the phosphorylated serine or threonine (By similarity). Tetramer composed of an alpha subunit, an alpha' subunit, one beta subunit and one beta' subunit (PubMed:12242279). Interacts with FACT subunits POB3 and SPT16 (PubMed:12242279). interacts with YTA7 (PubMed:22156209). Phosphorylated by alpha subunit. Present with 8120 molecules/cell in log phase SD medium. Belongs to the casein kinase 2 subunit beta family.