Abstract
Development of neural and sensory primordia at the early stages of embryogenesis depends on the activity of two B1 Sox transcription factors, Sox2 and Sox3. The embryonic expression patterns of the Sox2 and Sox3 genes are similar, yet they show gene-unique features. We screened for enhancers of the 231-kb genomic region encompassing Sox3 of chicken, and identified 13 new enhancers that showed activity in different domains of the neuro-sensory primordia. Combined with the three Sox3-proximal enhancers determined previously, at least 16 enhancers were involved in Sox3 regulation. Starting from the NP1 enhancer, more enhancers with different specificities are activated in sequence, resulting in complex overlapping patterns of enhancer activities. NP1 was activated in the caudal lateral epiblast adjacent to the posterior growing end of neural plate, and by the combined action of Wnt and Fgf signaling, similar to the Sox2 N1 enhancer involved in neural/mesodermal dichotomous cell lineage segregation. The Sox3 D5 enhancer and Sox2 N3 enhancer were also activated similarly in the diencephalon, optic vesicle and lens placode, suggesting analogies in their regulation. In general, however, the specificities of the enhancers were not identical between Sox3 and Sox2, including the cases of the NP1 and D5 enhancers.
Highlights
Group B1 Sox genes, Sox1, Sox2 and Sox3, encode HMG-domain transcription factors that show almost identical activity in the activation of target genes in transactivation assays [1±3], which is consistent with the model of their derivation from the same ancestral gene as a consequence of genome duplications [4]
The Sox3 gene is located on the X-chromosome in mammals, but it is autosomal in other vertebrate species
As the majority of enhancer sequences are included in the sequence blocks that are strongly conserved across the species (>60% DNA sequence identity over a length of 100 bp) [27], we compared the distribution of Conserved Sequence Blocks (CSBs)
Summary
Group B1 Sox genes, Sox, Sox and Sox, encode HMG-domain transcription factors that show almost identical activity in the activation of target genes in transactivation assays [1±3], which is consistent with the model of their derivation from the same ancestral gene as a consequence of genome duplications [4]. In embryos, these B1 Sox factors regulate neuro-sensory development [5] in a functionally redundant manner when their expression overlaps in tissue. Sox activity compensates for the loss of Sox expression in the epiblast of post-gastrulation embryos [18]
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