A rabbit reticulocyte factor which stimulates protein synthesis in several mammalian cell-free systems.

Rabbit reticulocyte lysate post-ribosomal supernatant is shown to stimulate protein synthesis in a variety of mammalian cell-free systems, particularly the less efficient systems, such as those from mouse liver, HeLa cells and heat-shocked L cells. This stimulation reflects an increase in the rate of initiation, and is not due to the presence of globin mRNA. The stimulatory activity is unstable to purification, but some conditions favouring stability have been identified and partial purification has been achieved. It is free of eIF-2, but possesses eIF-2B activity. Its purification properties suggest that it is distinct from previously characterized initiation factors, including eIF-2/eIF-2B complex, and its possible relationship to known initiation factors is discussed.