Targeted disruption of an H3-IV/H3-V gene pair causes increased expression of the remaining H3 genes in the chicken DT40 cell line.

The chicken H3 gene family contains 12 members (H3-I to H3-XII). Nine of them belong to two major histone gene clusters, but the genomic locations of the others are unknown. In the DT40 chicken B cell line, H3-IV and H3-V, which are located in inverted orientation and share a 3'-untranslated region of 531 bp, produce about 24% of the steady-state level of total mRNAs from all the H3 genes. To study the nature of these two genes, we carried out targeted integration into DT40 cells of constructs of the H3-IV/H3-V locus. Analysis of the stable transfectants showed that the growth rate of DT40 cells was unchanged in the absence of two copies of the H3-IV/H3-V locus. Furthermore, the remaining H3 genes were shown to be expressed more in the mutants lacking one or two H3-IV/H3-V alleles than DT40 cells, and the steady-state level of total H3 mRNAs remained constant in all the mutant cell lines tested. The levels of mRNAs from the H2B and H1 genes, which are located close to the H3 genes, were unchanged. These results demonstrate that the chicken H3 gene family has the ability to compensate for disruption of H3-IV/H3-V, which are normally responsible for about 24% of total H3 mRNAs.