ABSTRACT The halophilic archaeon Haloferax volcanii produces three different proteins (α1, α2, and β) that assemble into at least two 20S proteasome isoforms. This work reports the cloning and sequencing of two H. volcanii proteasome-activating nucleotidase (PAN) genes ( panA and panB ). The deduced PAN proteins were 60% identical with Walker A and B motifs and a second region of homology typical of AAA ATPases. The most significant region of divergence was the N terminus predicted to adopt a coiled-coil conformation involved in substrate recognition. Of the five proteasomal proteins, the α1, β, and PanA proteins were the most abundant. Differential regulation of all five genes was observed, with a four- to eightfold increase in mRNA levels as cells entered stationary phase. In parallel with this mRNA increase, the protein levels of PanB and α2 increased severalfold during the transition from exponential growth to stationary phase, suggesting that these protein levels are regulated at least in part by mechanisms that control transcript levels. In contrast, the β and PanA protein levels remained relatively constant, while the α1 protein levels exhibited only a modest increase. This lack of correlation between the mRNA and protein levels for α1, β, and PanA suggests posttranscriptional mechanisms are involved in regulating the levels of these major proteasomal proteins. Together these results support a model in which the cell regulates the ratio of the different 20S proteasome and PAN proteins to modulate the structure and ultimately the function of this central energy-dependent proteolytic system.