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So, to summarize what we know, and don't know, about RNase
P in Archaea:
- RNase P in (most) Archaea has an RNA that is of the same
type as in Bacteria, resembling them in sequence, structure,
and function. These RNAs are catalytically active, but depend
more on the proteins for stability than do the bacterial RNAs.
- RNase P enzymes in Archaea contain at least 4 proteins. These
proteins are homologs of 4 of the 9-12 eukaryotic nuclear proteins,
not the single small bacterial protein. The structures of these
proteins, what they contribute to the function of the enzyme
and how they do it, and the potential for additional protein
subunits, are under investigation.
- RNase P in Archaea is an evolutionary chimera of the bacterial
and nuclear enzymes; the RNA is bacterial, and the proteins are,
in essence, nuclear. This is a microcosm of the generality in
Archaea, resembling Bacteria in some ways and eukaryoties in
others. This also implies that the last common ancestor of all
3 Domains either had an RNA-only enzyme, and each of the two
initial branches (Bacteria on one hand, Archaea and eukaryoties
on the other) solved the protein problem independently, or that
the enzyme contained archaeal-like proteins in the LCA and these
were elaborated in the case of eukaryotes, but simplified in
the case of Bacteria. (The structures of the proteins will probably
distinguish between these two posssibilities.)
- Although most archaeal RNase P enzymes contain a type A RNA,
those of the methanococci and Archaeoglobus are of type
M; how these enzymes compensate for the absence of essential
elements of the RNA structure that are directly involved in substrate
recognition remains to be determined.
- In addition, the RNase P RNAs of Pyrobaculum are missing most of the S-domain. But since we haven't been able to show catalytic activity from either the RNA
nor from cell extracts (i.e. the holoenzyme), this RNase P may have very different substrate requirements - and although it is clearly an RNase P homolog, may not function as other RNase P's do.
