Ribosome Recycling Factor

Ribosome Recycling Factor (RRF) is a protein found in bacterial cells as well as eukaryotic organelles, specifically mitochondria and chloroplasts. It functions to recycle ribosomes after completion of protein synthesis.

Discovery
RRF was disocvered in the early 1970s by the groundbreaking work   of Akira Kaji and Akikazu Hiroshima at the University of Pennsylvania. Their work described the requirement for two protein factors to release ribosomes from mRNA. These two factors were identified as RRF, an unknown protein until then, and Elongation Factor G (EF-G), a protein already identified and known to function in protein synthesis. RRF was originally called Ribosome Releasing Factor but is now called Ribosome Recycling Factor.

Function
Recent evidence suggests RRF may accomplish the recycling of ribosomes by splitting ribosomes into subunits, thereby releasing the bound mRNA.

Loss of RRF Function:
 * In Bacteria (specifically Escherichia coli), loss of the gene encoding RRF is deleterious . This makes RRF a possible target for new antibacterial drugs.
 * Yeast mitochondrial RRF (mtRRF) is encoded by a gene in the cell nucleus. Loss of function of this gene leads to mitochondrial genome instability and respiratory incompetence.

Structure of RRF and Binding to Ribosomes
The crystal structure of RRF was first determined by X-ray diffraction in 1999. The most striking revelation was that RRF is a near-perfect structural mimic of tRNA, in both size and dimensions. One view of RRF can be seen here.

Despite the tRNA-mimicry, RRF binds to ribosomes quite differently from the way tRNA does. It has been suggested that ribosomes bind proteins (or protein domain) of similar shape and size to tRNA, and this, rather than function, explains the observed structural mimicry.