The cAMP-dependent transcription of carbonic anhydrase II (CAII) is one of the last differentiation steps in circulating red blood cells (RBCs) of the chicken embryo. Although the translational machinery is still mainly engaged in globin synthesis, the abundant CAII mRNA becomes efficiently translated. The induction of CAII is accompanied by the expression of the anti-proliferative TOB protein. Among others, TOB is known to regulate the mRNA metabolism by binding to the poly(A)-binding protein PABP. - The investigation was aimed to study the TOB induction, regulation, and localization in chicken embryonic RBCs in order to correlate the presence of TOB to the mRNA expression and translation of CAII and b-globin: In vitro and in vivo, post-mitotic definitive but not early primitive RBCs, transiently express the TOB protein after transcriptional activation by protein kinase A-dependent CREB phosphorylation. The transient TOB expression correlates with a decreased PABP binding to the polysomes and results in a decreased translation of b-globin in favor of caII while a prolonged TOB expression destabilizes the CAII and the b-globin mRNA. From these data, we suggest that the transient TOB expression might destabilize existing translation complexes in order to provide the translation factors for translation of the newly synthesized CAII mRNA.