MOLECULAR-CLONING OF MOUSE SOMATIC AND TESTIS-SPECIFIC H2B HISTONE GENES CONTAINING A METHYLATED CPG ISLAND

We have isolated a mouse testis-specific H2B histone gene based on the unusual methylation of the CpG island of rat testis-specific H2B gene in somatic tissues, After digestion of genomic DNA with the methylati on-sensitive restriction enzyme Hha I, we found that, among 10-20 copi es of mouse H2B histone genes, at least three copies are methylated in somatic tissues, but not in testis, Cloning and sequence analysis of two methylated H2B genes revealed that one gene, MTH2B, is strikingly similar to the testis-specific histone H2B (TH2B) gene of rat and the other, psH2B, is a pseudogene of the somatic-type H2B gene, Northern b lot analysis revealed that the expression of the MTH2B gene is testis- specific. During spermatogenesis, the MTH2B gene is expressed predomin antly in pachytene spermatocytes, as observed in the expression of rat TH2B gene, Interestingly, the MTH2B gene is largely unmethylated in e mbryonic stem cells, but methylated in F9 embryonal carcinoma cells, T he psH2B pseudogene is methylated in somatic tissues and F9 cells, but only partially methylated in embryonic stem cells, Methylation of the psH2B pseudogene seems to be attributed to its location within the co ntext of repetitive sequences including the B1 element. The unmethylat ion of both H2B histone genes in the testis explains how CpG islands o f those histone genes can be maintained during evolution despite heavy methylation in somatic tissues.