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FIGURE 8. Model for the repression of the oocyte-type 5 S rRNA genes following the midblastula transition. A, repression of the oocyte-type genes is largely due to histone H1-dependent positioning of a nucleosome over the binding site for TFIIIA (nucleotides 45–96). The A:T-rich sequence flanking the 3′ end of the gene, which is the binding site for the linker histone, is required for repression. In addition, ∼1% of the oocyte-type genes are bound by TFIIIA, which can interact with PIAS2b and the CtBP corepressor complex. This complex may be stabilized by an interaction between the SAP domain of PIAS2b and the A:T-rich sequence that immediately flanks the 3′ end of the oocyte-type genes. B, the somatic-type genes are flanked by G:C-rich sequences, providing no contact site for PIAS2b. Histone octamers are weakly positioned on the somatic-type gene, and the binding site for TFIIIA is exposed. Binding of the transcription factor is sufficient to reposition the octamer that possibly leads to dissociation of the H1 linker histone (18). Image published in: Malik MQ et al. (2014) Copyright © 2014. Image reproduced with permission of the publisher and the copyright holder. This is an Open Access article distributed under the terms of the Creative Commons Attribution License. Larger Image Printer Friendly View

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