In differentiating red blood cells (RBCs) of the chick embryo, the synthesis of carbonic anhydrase (CAII) and pyrimidine 5'-nucleotidase (P5N-I) is triggered by the hypoxic mediators norepinephrine and adenosine via receptor-mediated cAMP formation. The process is accompanied by the induction of IFR1 and TOB which are putative regulators of transcription or translation in different cell types. The present investigation studied the erythroid TOB and IFR1 expression: mRNA and protein are up-regulated in post-mitotic RBCs from D11-19 treated with cAMP-elevating agonists. In contrast, immature RBCs of early embryos (D5-7) fail to synthesize a significant amount of IFR1/TOB. In D11 RBCs, TOB and IFR1 are cytosolic proteins with different half-lives JOB < 4 h, IFR1 > 12 h). Cytosolic fractionation characterized TOB as a free soluble protein while the abundant IFR1 (c(max)similar to 3 mu M) is completely associated with the ribosomal fraction. A putative function of both proteins as translational regulators is discussed. (c) 2007 Elsevier Inc. All rights reserved.