Phosphorylation of ribosomal proteins during maturation of Xenopus laevis oocytes

The formation of an active maturation‐promoting factor and a strong burst of protein phosphorylation occur together at germinal vesicle breakdown in Xenopus oocytes undergoing maturation after progesterone Treatment. Here we show that one of the sites of this phosphorylation is the 40‐S ribosomal protein S6. This observation is discussed in relation to the known presence of a maturation promoting factor in somatic cells during the G2‐M transition and to the increase in S6 phosphorylation observed in cells treated with mitogenic stimuli.