| Figure 1 Protein production and the effects of the
Sup35 protein and its modified form, the [PSI+] prion,
in yeast. a, In cells with normal Sup35 protein, translation of
the messenger RNA sequence into a protein sequence by the ribosome is normally
terminated by a stop codon. Termination requires an interaction between
Sup35 and other proteins such as Sup45. b, In [PSI+]
cells, the Sup35 protein takes up the prion conformation and forms aggregates
that fail to interact properly with the termination complex. Stop codons
are therefore sometimes missed, producing proteins with an extra segment.
True and Lindquist3 argue that the prion is maintained by natural
selection because the variant proteins it induces sometimes increase yeast-cell
ability to survive environmental change. (Graphic modified from material
provided by Julie Feder.) |
