Changes in intracellular levels of Ap3A and Ap4A in cysts and larvae of Artemia do not correlate with changes in protein synthesis after heat-shock.

Artemia larvae respond to a brief heat-shock between 28 degrees and 40 degrees C with an increase in the synthesis of two groups of proteins of Mr 68,000 and 89,000. At 40 degrees C synthesis of all other proteins is strongly repressed. Cysts, which are naturally thermotolerant, synthesise both heat-shock proteins at temperatures up to 47 degrees C but maintain normal protein synthesis. During pre-emergence development, Ap3A is present in cysts at a concentration twice that of Ap4A. The maximum level of 7.6 pmol/10(6) cells is reached shortly before hatching of the larvae. After hatching, the levels of both nucleotides decline. A 40 degrees C heat-shock produces a 1.8-fold increase in both nucleotides within 20 min in cysts and larvae. A 2.8-fold increase results from a 47 degrees C heat-shock to cysts. The rates of increase parallel but do not precede the increases in the heat-shock proteins. Since non-heat-shocked cysts possess higher levels of Ap3A and Ap4A than do heat-shocked larvae, the observed heat-induced changes in gene expression cannot be explained simply in terms of the intracellular concentrations of these nucleotides.