Characterization of chicken CENP-A and comparative sequence analysis of vertebrate centromere-specific histone H3-like proteins.

Centromere protein A (CENP-A) is a centromere-specific histone H3 variant conserved amongst all eukaryotes. We have isolated the chicken gene for CENP-A (GgCENP-A). It encodes a 131-amino-acid polypeptide that possesses an average identity of 54% with human CENP-A, reaching 69% in the histone-fold domain. The gene spans 1.7 kb of genomic DNA and contains four exons that range in size from 78 to 186 bp. The exon/intron organisation of the chicken gene is conserved with its mammalian counterparts in the carboxy-terminal histone-fold domain (exons 2 to 4), consistent with the strong conservation of this domain at the amino acid level. Sequence analysis of the chicken CENP-A locus revealed that the gene is located within the class III genes of the major histocompatibility complex (MHC), and extended the previously defined limit of the compact chicken MHC complex. We compared the sequences of CENP-A from mammals, chicken and fishes and thereby identified conserved motifs in the otherwise variable amino-terminal tail that may be important for functional reasons. We also identified evolutionarily variable regions within the conserved histone-fold domain. We found that loop 1 between the first and second alpha-helix is the region that diverged most widely. This finding is in agreement with evolutionary studies in Drosophila species, and suggests that this domain could play a role in species-specific centromere targeting of CENP-A. In addition, protein sequence comparison of several vertebrate species revealed that the RT-PCR strategy we have developed for isolating the chicken centromeric histone H3 variant gene should be applicable to the isolation of CENP-A from a wide range of vertebrates.