How Do Linker Histones Mediate Differential Gene Expression?

In developing Xenopus laevis embryos the multiple-copy oocyte-type 5S RNA genes are progressively shut down. Results presented in three recent articles 1-3 together demonstrate that replacement of the cleavage stage linker histone B4 by somatic H1 leads to chromatosomes positioned directly over these genes and adjacent sequences so as to occlude the binding site for the critical transcription factor TFIIIA. In contrast, on the somatic-type 5S genes the somatic H1 positions chromatosomes about 65 bp further upstream, thereby leaving the TFIIIA binding site exposed and the genes active. The somatic linker histone thus functions as a specific gene repressor.