Role of cyclic AMP in the control of cell-specific gene expression

Genes have to be expressed in specific cell types at appropriate times of development dependent on external signals. cAMP signaling occurs in all cells, thus raising the question of how this signal transduction pattern is integrated into mechanisms determining cell-specific gene expression. We have analyzed expression of the tyrosine aminotransferase gene as a model to study the basis of this cell type specificity of hormone induction. We found that cell-type-specific expression is generated by combined action of cAMP signal-dependent and liver cell-specific transcription factors. The interdependence of the cAMP response element and an element determining liver cell specificity enables a gene to respond to an ubiquitous signal in a cell-specific manner

The stereochemistry of the formation of the methyl group in the glutamate mutase-catalysed reaction in Clostridium tetanomorphum

AbstractThe adenosylcobalamin-dependent enzyme glutamate mutase from Clostridium tetanomorphum catalyses the reversible rearrangement of (2S)-glutamate to (2S,3S)-3-methylaspartate. In this conversion 6 carbon centers are involved. The stereochemistry of 4 has been elucidated whereas the formation of the methyl group from the methylene group remains to be established. To solve this problem, (2S,3R)- and (2S,3S)-[3,3-2H1,3H]glutamates were prepared via the 2-oxo[3,3-2H2 or 3H]glutarates by incubation with isocitrate dehydrogenase in deuterium oxide or tritiated water. The labelled glutamates were fermented with growing cells of C. tetanomorphum to butyrate and acetate. Butyrate was further degraded to acetate in which methyl group over 90% of the tritium of the starting glutamate was retained. The chirality of the acetates was determined with malate synthase and fumarase. In both samples complete racemisation was found. This result confirms the rule that racemisation occurs in all adenosylcobalamin-dependent rearrangements in which methyl groups are formed. A methylene radical as intermediate could explain these observations. In a control experiment inversion of configuration in the formation of the methine group of (2S,3S)-3-methylaspartate from the methylene group of (2S)-glutamate was confirmed. Glutamates stereospecifically labelled at C-4 were synthesized from chiral acetates via citrate