The human proto-oncogene c-myc encodes two proteins, c-Myc1 and c-Myc2, from two initiation codons, CUG and AUG, respectively. It is also transcribed from four alternative promoters (P0, P1, P2, and P3), giving rise to different RNA 5-leader sequences, the long sizes of which suggest that they must be inefficiently translated by the classical ribosome scanning mechanism. Here we have examined the influence of three c-myc mRNA 5-leaders on the translation of chimeric myc-CAT mRNAs. We observed that in the reticulocyte rabbit lysate, these 5-leaders lead to cap-independent translation initiation. To determine whether this kind of initiation resulted from the presence of an internal ribosome entry site (IRES), COS-7 cells were transfected with bicistronic vectors containing the different c-myc 5-leaders in the intercistronic region. An IRES was identified, requiring elements located within the P2 leader, between nucleotides ؊363 and ؊94 upstream from the CUG start codon. This is the first demonstration of the existence of IRES-dependent translation for a proto-oncogene. This IRES could be a translation enhancer, allowing activation of c-myc expression under the control of trans-acting factors and in response to specific cell stimuli.

The genetic basis for the development of brainstem neurons that generate respiratory rhythm is unknown. Here we show that mice deficient for the transcription factor MafB die from central apnea at birth and are defective for respiratory rhythmogenesis in vitro. MafB is expressed in a subpopulation of neurons in the preBötzinger complex (preBötC), a putative principal site of rhythmogenesis. Brainstems from Mafb(-/-) mice are insensitive to preBötC electrolytic lesion or stimulation and modulation of rhythmogenesis by hypoxia or peptidergic input. Furthermore, in Mafb(-/-) mice the preBötC, but not major neuromodulatory groups, presents severe anatomical defects with loss of cellularity. Our results show an essential role of MafB in central respiratory control, possibly involving the specification of rhythmogenic preBötC neurons.

Several proteins that may regulate c-myc mRNA posttranscriptionally were previously isolated and characterized. Two of them, HuR and AUF1, bind speci®cally to the 3' untranslated region (UTR) of c-myc mRNA. Because c-myc is regulated post-transcriptionally in various mouse tissues, including quiescent tissues, fetal liver and regenerating liver, we investigated whether HuR and AUF1 expression was also regulated in these tissues. Concerning AUF1, we analysed the expression of various mRNA and protein isoforms. We discovered a new AUF1 mRNA variant with a long AU-rich 3' UTR. We show that AUF1 expression, regardless of the RNA isoform considered, and HuR mRNA expression parallel c-myc expression in quiescent tissues and during liver development; their expression is high in lymphoid tissues and fetal liver and low in adult liver. However, no upregulation of HuR or AUF1 accompanies the upregulation of c-myc mRNA following partial hepatectomy. We discuss our results in relation to the current hypothesis that HuR and AUF1 act as mRNA destabilizing factors.