So, we know something about 4 archaeal RNase P proteins, but is that all there are? It turns out that Archea have a homolog of the eukaryotic Sm protein that is preferentially associated with the precursor of RNase P RNA in Archaeoglobus fulgidus; this presumably reflects RNase P RNA biogenesis rather than the mature, active enzyme. On the other hand, the implication of our ability to reconstitute normal activity from these 4 proteins and the RNA is that these are the only essential subunits of the enzyme. But there could be other subunits that contribute less directly, and there is some reason to think this might be so. The bouyant density of the M. thermoautotrophicus RNase P holoenzyme in cesium sulfate is 1.42, which translates into an RNA:protein ratio of about 0.96. The mass of the RNA is 94 kDa, so the total mass of the proteins ought to be about 98 kDa. The 4 proteins add up to only 71 kDa - 23 kDa of protein is missing. This might just mean that one or more of the proteins is present in more than one copy per RNA, and indeed we saw evidence for dimerization of Mth11 (its apparent ability to interact with itself). But even if we account for 2 molecules of Mth11, about 10 kDa of protein is AWOL.