The modulator is a constitutive enhancer of a developmentally regulated sea urchin histone H2A gene

Abstract

Going back to the late 1970s and early 1980s, we trace the Xenopus oocyte microinjection experiments that led to the emergence of the concept of "modulator". The finding that the modulator could transactivate transcription from far upstream and in either orientation suggested that a new genetic element, different from the classical prokaryotic promoter sequences, had been discovered. This particular enhancer transactivates transcription of the sea urchin early (alpha) histone H2A gene which is regulated in early sea urchin development. We summarise the data from sea urchin microinjection experiments that confirm and extend the results obtained with Xenopus oocytes. We conclude that the H2A enhancer is bipartite, is located approx. 100 bp upstream of the TATAAATA box in the H2A gene of two sea urchin species and enhances transcription when placed at a position far upstream or far downstream of the gene unless an insulator intervenes between enhancer and promoter. Evidence from microinjection experiments with sea urchin embryos suggests that the developmental control of H2A expression resides not with the enhancer, which is constitutively active, but with a striking chromatin structure with two positioned nucleosomes near the 3' end of the gene. Within this structure, there is an insulator element.

Constitutive Promoter Occupancy by the MBF-1 Activator and Chromatin Modification of the Developmental Regulated Sea Urchin α-H2A Histone Gene

“…28 These findings led to the paradoxical conclusion that the modulator is a constitutive enhancer of a developmentally regulated sea urchin histone α-H2A gene. 19 The results described here extend these observations. We have in fact presented compelling evidence that the MBF-1 activator is constitutively associated to the chromatin, and yet, the H2A gene is down-regulated at gastrula stage.…”

“…Thereafter these genes are repressed and never expressed again in the life cycle of the animal. [18][19][20][21] The heritable repressed state of the α-histone genes correlates with changes of chromatin organization. During the period of maximum transcription the chromatin of these genes, probed with the coding or spacer regions, shows a highly irregular nucleosomal package, with a randomized nucleosome spacing, and hypersensitivity to nuclease digestion.…”

Chromatin remodeling during sea urchin early development: molecular determinants for pronuclei formation and transcriptional activation

“…Recent evidence from microinjection experiments with sea urchin embryos suggest that the developmental control of H2A expression is not associated with this enhancer element, which is constitutively active, but with a striking chromatin organization at the 3V-end of the H2A gene. This chromatin structure involves two positioned nucleosomes which comprise an insulator element (reviewed by Spinelli and Birnstiel, 2002).…”

Down-regulation of Early Sea Urchin Histone H2A Gene Relies on cis Regulative Sequences Located in the 5′ and 3′ Regions and Including the Enhancer Blocker sns

“…1 The sea urchin early (or a) histone gene family represents a model system for the study of transcription regulation during development. 2 The sea urchin genome contains four distinct histone gene families, which are sequentially expressed during ontogeny and which code for a repertoire of histone variants. [3][4][5] The a-subtype histone genes are organized as tandem arrays of a repeating unit containing the five genes.…”