Mutations of survival of the motor neuron gene (SMN1) are responsible for spinal muscular atrophy (SMA), a common genetic cause of death in childhood. The cellular mechanism by which mutations of SMN1 are responsible for the selective neuromuscular defect and motor neuron cell degeneration observed in SMA has not been described. We have previously generated mice carrying a homozygous deletion of Smn exon 7 directed to neurons. We report here that these mutant mice display a dramatic and progressive loss of motor axons involving both proximal and terminal regions, in agreement with the skeletal muscle denervation process and disease progression. Moreover, we found massive accumulation of neurofilaments, including phosphorylated forms, in terminal axons of the remaining neuromuscular junctions. This aberrant cytoskeletal organization of synaptic terminals was associated with reduction of branched structures of the postsynaptic apparatus and defect of axonal sprouting in mutant mice. Together, these findings may be responsible for severe motor neuron dysfunction, and suggest that loss of motor neuron cell bodies results from a 'dying-back' axonopathy in SMA. Smn mutant mice should represent a valuable model for elucidating the pathway linking Smn to cytoskeletal organization.

In Saccharomyces cerevisiae the nicotinic acid moiety of NAD + can be synthesized from tryptophan using the kynurenine pathway or incorporated directly using nicotinate phosphoribosyl transferase (NPT1). We have identified the genes that encode the enzymes of the kynurenine pathway and for BNA5 (YLR231c) and BNA6 (YFR047c) confirmed that they encode kynureninase and quinolinate phosphoribosyl transferase respectively. We show that deletion of genes encoding kynurenine pathway enzymes are co-lethal with the v vnpt1, demonstrating that no other pathway for the synthesis of nicotinic acid exists in S. cerevisiae. Also, we show that under anaerobic conditions S. cerevisiae is a nicotinic acid auxotroph. ß

Carbon catabolite repression is mediated in Aspergillus nidulans by the negative acting protein CreA. The CreA repressor plays a major role in the control of the expression of the alc regulon, encoding proteins required for the ethanol utilization pathway. It represses directly, at the transcriptional level, the specific transacting gene alcR, the two structural genes alcA and aldA, and other alc genes in all physiological growth conditions. Among the seven putative CreA sites identified in the alcA promoter region, we have determined the CreA functional targets in AlcR constitutive and derepressed genetic backgrounds. Two different divergent CreA sites, of which one overlaps a functional AlcR inverted repeat site, are largely responsible for alcA repression. Totally derepressed alcA expression is achieved when these two CreA sites are disrupted in addition to another single site, which overlaps the functional palindromic induction target. The fact that derepression is always associated with alcA overexpression is consistent with a competition model between AlcR and CreA for their cognate targets in the same region of the alcA promoter.Our results also indicate that the CreA repressor is necessary and sufficient for the total repression of the alcA gene.

Expression of the structural genes for alcohol and aldehyde dehydrogenase, alcA and aldA, respectively, enables the fungus Aspergillus nidulans to grow on ethanol. The pathway-specific transcriptional activator AlcR mediates the induction of ethanol catabolism in the presence of a coinducing compound. Ethanol catabolism is further subject to negative control mediated by the general carbon catabolite repressor CreA. Here we show that, in contrast to alcA and alcR, the aldA gene is not directly subject to CreA repression. A single cisacting element mediates AlcR activation of aldA. Furthermore, we show that the induction of the alc gene system is linked to in situ aldehyde dehydrogenase activity. In aldA loss-of-function mutants, the alc genes are induced under normally noninducing conditions. This pseudo-constitutive expression correlates with the nature of the mutations, suggesting that this feature is caused by an intracellular accumulation of a coinducing compound. Conversely, constitutive overexpression of aldA results in suppression of induction in the presence of ethanol. This shows unambiguously that acetaldehyde is the sole physiological inducer of ethanol catabolism. We hypothesize that the intracellular acetaldehyde concentration is the critical factor governing the induction of the alc gene system.