Proteasomes (multi-protease complexes) are composed of approximately 15 non-identical subunits of similar sizes (molecular weight = 21-32 kDa), but different charges (isoelectric point = 4-9). Previously, we deduced the primary structures of 6 subunits of rat proteasomes by recombinant DNA techniques. In this paper we report the nucleotide sequences of 4 other subunits, rIOTA, rZETA, rDELTA, and rRING12, determined from cDNA clones isolated by screening a rat H4TG hepatoma cell cDNA library with the cDNAs of their human counterparts as probes. The polypeptides deduced from their nucleotide sequences consisted of 246, 241, 202, and 219 amino acid residues with calculated molecular weights of 27,399, 26,391, 21,649, and 23,324, and calculated isoelectric points of 6.37, 4.65, 4.84, and 4.70, respectively. These results and previous findings indicate that the primary structures of the subunits of rat proteasomes show considerably high inter-subunit homology, but can be classified into apparently distinct sub-groups, suggesting that rat proteasome genes form a multi-gene family with the same evolutionary origin, but have diverged during evolution to acquire possibly subunit-specific functions.