A single copy of linker H1 genes is enough for proliferation of the DT40 chicken B cell line, and linker H1 variants participate in regulation of gene expression

Abstract

Background: There is general agreement that large numbers of histone H1 are necessary for maintenance of the higher order structure of chromatin in higher eukaryotes. The chicken H1 gene family comprises six members per haploid genome, the total copy number being 12, and they encode six H1 variants which are considerably different from each other in amino acid sequence. We recently established that in two chicken DT40 mutants (1/2delta110kb and delta57kb), which lack, respectively, one allele of the gene cluster of 110 kb carrying six H1 genes, plus 33 core histone genes, and two copies each of four of the six H1 genes included in an approximately 57 kb segment of the cluster, expression of the remaining H1 genes is increased, resulting in constant steady-state levels of total H1 mRNAs. These results gave rise to the simple questions of how many H1 genes and how many H1 variants, at minimum, are necessary for the viability of DT40 cells.

Results: We generated two DT40 mutants, delta10/12H1 and delta11/12H1, which are devoid, respectively, of two copies each of five H1 genes, and those plus a single copy of the last H1 gene, in addition to 17 core histone genes. Analyses involving a RNase protection assay, SDS-PAGE and acid-urea-PAGE revealed, not only that in the delta10/12H1 mutant the steady-state levels of total H1 mRNAs and the amounts of histone H1 were not changed, but also that in the delta11/12H1 mutant both were approximately one-half the normal levels, and the amounts of HMG proteins were increased about twofold. No alteration in the growth rate or global chromatin structure was observed in either mutant. On the other hand, the protein patterns on 2D-PAGE of the delta11/12H1 mutant were definitely distinct from those of the wild-type cell line.

Conclusion: These results indicate not only that a lack of five of the six H1 variants causes changes in the protein patterns, but also that only a single copy of the H1 genes is enough for cell proliferation.