The peptidyl transferase center (PTC) of all ribosomes comprises catalytic RNA. Evidence in favor of “the ribosome is a ribozyme.”
But I would welcome the comments of Art, John Mercer, T. aqua, and others who care about this question, on the following exchange. The speakers here are George Church, Craig Venter, and the late Robert Shapiro:
CHURCH: But isn’t it the case that, if we take all the life forms we have so far, isn’t the minimum for the ribosome about 53 proteins and 3 polynucleotides? And hasn’t that kind of already reached a plateau where adding more genomes doesn’t reduce that number of proteins?
VENTER: Below ribosomes, yes: you certainly can’t get below that. But you have to have self-replication.
CHURCH: But that’s what we need to do — otherwise they’ll call it irreducible complexity. If you say you can’t get below a ribosome, we’re in trouble, right? We have to find a ribosome that can do its trick with less than 53 proteins.
VENTER: In the RNA world, you didn’t need ribosomes.
CHURCH: But we need to construct that. Nobody has constructed a ribosome that works well without proteins.
SHAPIRO: I can only suggest that a ribosome forming spontaneously has about the same probability as an eye forming spontaneously.
CHURCH: It won’t form spontaneously; we’ll do it bit by bit.
SHAPIRO: Both are obviously products of long evolution of preexisting life through the process of trial and error.
CHURCH: But none of us has recreated that any.
SHAPIRO: There must have been much more primitive ways of putting together
CHURCH: But prove it.
(source: Life: What A Concept! | Edge.org )
John Harshman, in another thread, said that someone had digested away all the proteins from a ribosome in vitro, and what remained (the RNAs) still functioned. I would be very interested in seeing that publication, if anyone can find it (I couldn’t).
I don’t want to accuse Art and John Mercer of begging the question, but their statements in another thread, namely, that there is no question that the ribosome is a ribozyme, remind me of this imaginary exchange:
Joe: An automobile engine is just a gas fire.
Moe: Say what?
Joe: Combine oxygen and gasoline, provide a spark – bingo, power. The rest is just details and decorations.