Transcript levels and translational control of hsp70 synthesis in Xenopus oocytes

Until recently it was believed that Xenopus oocytes respond to heat shock by synthesizing the 70-kD heat shock protein hsp70 and that, uniquely amongst animal cell types, this response is mediated entirely at the translational level. This view has now been challenged and we present data that reevaluate the involvement of translational control in the heat shock response of Xenopus oocytes. RNase mapping shows that up to 13 pg of hsp70A and hsp70B mRNA are accumulated by fully grown oocytes in the absence of heat shock. These transcripts are retained stably during maturation, fertilization, early cleavage, and following heat shock. However, no hsp70 protein synthesis can be detected by two-dimensional polyacrylamide gel analysis of [35S]methionine-labeled proteins from completely defolliculated oocytes, either before or during heat shock. Oocytes injected with hsp70A DNA rapidly accumulate high levels of hsp70 mRNA in their cytoplasm at normal temperature. During heat shock these oocytes accumulate more transcripts, but they remain in the nucleus and cytoplasmic levels remain constant. Translation of hsp70 from these transcripts is readily detectable at non-heat shock and heat shock temperatures. We conclude that (1) "exogenous" hsp70 transcripts are efficiently translated and not masked at normal temperatures in oocytes, and (2) oocytes are able to selectively translate hsp70 mRNA during heat shock.