Spiegelman Monster

An American microbiologist, Sol Spiegelman, created the smallest possible replicating organism. He did this by taking a small virus, Qb, and giving it replicase so it could replicate. Then he added nucleotides and let it start to slowly mutate. Eventually it went from its original size, 4,500 base pairs, to 220. This little "monster" could replicate extremely fast due to its size and was the smallest organism while it was "alive."

I, personally, doubt the validity of the story. 220 base pairs? That would only be enough for replicase. What about RNA polymerase or other vital proteins? In any case I think it's an interesting idea. But this all raises the question of what is the smallest organism (in theory). So then, what is the smallest theoretical organism, or does anyone think the Spiegelman account is true?

(In case you want to read an account of the story: Spiegelman's Monster)