In vertebrates, the embryonic hindbrain is transiently subdivided along its anterior-posterior (A-P) axis into 8 well defined segments termed rhombomeres (r1-8). Each rhombomere represents a true cellular compartment in transcriptional profile, lineage restriction and neuronal organization. Thus, the vertebrate hindbrain provides a beautiful model for studying mechanisms of anterior-posterior patterning, signal transduction and interpretation, initiation and maintenance of transcriptional profiles, cell sorting and border formation. The Kreisler/MafB gene, which encodes a basic leucine zipper (bZIP) transcription factor that regulates some Hox genes, is one of the first genes to be expressed segmentally in the hindbrain, and is subject to a dynamic and complex regulatory process. However, unlike the Hox genes, Kreisler/MafB is not located within a large cluster of genes and therefore provides a simple system for dissecting the molecular mechanisms involved in hindbrain compartmentalization. In dissecting the mechanisms that govern Kreisler/MafB regulation, we have identified the S5 regulatory element that directs early MafB expression in the future r5-r6 domain. We have found a binding site within S5 that is specific for the Variant Hepatocyte Nuclear Factor 1 (vHNF1) to be essential, but not sufficient for early induction of r5-r6-specific expression. Thus, early inductive events that initiate MafB expression are clearly distinct from later acting ones that modulate its expression levels. Using mouse mutants, we have shown that MafB is dependent on the M33 polycomb protein and other mechanisms of chromatin remodeling. We then utilized transgenic flies and mice as well as binding assays to identify and validate a PcG/trxG response element (PRE), PRE1 which acts to reorganize the surrounding chromatin, regulating S5-dependent expression. To our knowledge, PRE1 is the first validated vertebrate PcG/trxG response element. Thus, PRE1 provides a springboard for further exploration of the mechanisms governing chromatin remodeling.

In vertebrates, the embryonic hindbrain is transiently subdivided along its anterior-posterior (A-P) axis into 8 well defined segments termed rhombomeres (r1-8). Each rhombomere represents a true cellular compartment in transcriptional profile, lineage restriction and neuronal organization. Thus, the vertebrate hindbrain provides a beautiful model for studying mechanisms of anterior-posterior patterning, signal transduction and interpretation, initiation and maintenance of transcriptional profiles, cell sorting and border formation. The Kreisler/MafB gene, which encodes a basic leucine zipper (bZIP) transcription factor that regulates some Hox genes, is one of the first genes to be expressed segmentally in the hindbrain, and is subject to a dynamic and complex regulatory process. However, unlike the Hox genes, Kreisler/MafB is not located within a large cluster of genes and therefore provides a simple system for dissecting the molecular mechanisms involved in hindbrain compartmentalization. In dissecting the mechanisms that govern Kreisler/MafB regulation, we have identified the S5 regulatory element that directs early MafB expression in the future r5-r6 domain. We have found a binding site within S5 that is specific for the Variant Hepatocyte Nuclear Factor 1 (vHNF1) to be essential, but not sufficient for early induction of r5-r6-specific expression. Thus, early inductive events that initiate MafB expression are clearly distinct from later acting ones that modulate its expression levels. Using mouse mutants, we have shown that MafB is dependent on the M33 polycomb protein and other mechanisms of chromatin remodeling. We then utilized transgenic flies and mice as well as binding assays to identify and validate a PcG/trxG response element (PRE), PRE1 which acts to reorganize the surrounding chromatin, regulating S5-dependent expression. To our knowledge, PRE1 is the first validated vertebrate PcG/trxG response element. Thus, PRE1 provides a springboard for further exploration of the mechanisms governing chromatin remodeling.