Results The ribosome is the missing guanine-nucleotide exchange factor for EF-G It has been reported that EF-G from Escherichia coli binds to GTP with ten-fold lower affinity than it binds to GDP [12]. On the assumption that there is a ten-fold excess of GTP over GDP in the cytoplasm and rapid nucleotide exchange on free EF-G, it was suggested that the rate-limiting step of guanine-nucleotide exchange in the EF-G cycle is the disso- ciation of EF-G•GDP from the postT ribosome [3].