Nanometer scale pores similar in size to the entrance of the ribosomal exit cavity are a common feature of large RNAs.

The highly conserved peptidyl transferase center (PTC) of the ribosome contains an RNA pore that serves as the entrance to the exit tunnel. Analysis of available ribosome crystal structures has revealed the presence of multiple additional well-defined pores of comparable size in the ribosomal (rRNA) RNAs. These typically have dimensions of 1-2 nm, with a total area of ∼100 Å(2) or more, and most are associated with one or more ribosomal proteins. The PTC example and the other rRNA pores result from the packing of helices. However, in the non-PTC cases the nitrogenous bases do not protrude into the pore, thereby limiting the potential for hydrogen bonding within the pore. Instead, it is the RNA backbone that largely defines the pore likely resulting in a negatively charged environment. In many but not all cases, ribosomal proteins are associated with the pores to a greater or lesser extent. With the exception of the PTC case, the large subunit pores are not found in what are thought to be the evolutionarily oldest regions of the 23S rRNA. The unusual nature of the PTC pore may reflect a history of being created by hybridization between two or more RNAs early in evolution rather than simple folding of a single RNA. An initial survey of nonribosomal RNA crystal structures revealed additional pores, thereby showing that they are likely a general feature of RNA tertiary structure.