Figure 1 : Schematic folding landscape of group-I intron RNAs.

From: Indifferent chaperones

Figure 1

In this representation, valleys indicate structures of varying stability — the deeper a valley, the lower the energy of the structure and the more stable it is. The ridges between valleys correspond to rates of inter-conversion between neighbouring structures; high ridges indicate slow rates. U represents an unfolded RNA, Ia and Ib are intermediate structures, M is a misfolded and C a correctly folded RNA. a, Left to their own devices, most group-I intron RNAs first form M, which is less stable than C but forms more quickly. However, M converts back into Ib and Ia at an appreciable rate, allowing further rounds of folding into both C and M. Although C can also convert back to Ib, this process is slow. So with time, C accumulates. b, Bhaskaran and Russell3 show that CYT-19 alters the RNA-folding landscape in an ATP-dependent manner. By unfolding both M and C, new pathways for structural conversions are opened that presumably lead to Ia/Ib (red arrows). The formation of the RNA species depends on the rates at which they can interconvert. This represents a kinetically controlled steady state, in which less stable structures can accumulate. The levels of ATP determine and maintain the topology of the folding landscape, and in the absence of ATP the landscape changes back to that seen in a.