Orangutan alpha-satellite monomers are closely related to the human consensus sequence

Alpha-satellite is a family of tandemly repeated DNA found at the centromeric regions of all human and primate chromosomes. Human alpha-satellite subsets are largely chromosome-specific and have been further grouped into four suprachromosomal families (SFs), each characterized by a unique set of monomeric types. Although chimpanzee and gorilla alpha-satellites share sufficient sequence similarity to fit the established SFs, the assumption that the derived human alpha-satellite consensus and monomeric types represent the sequence of ancestral repeats remains unestablished. By using oligonucleotide primers specific for a conserved region of human alpha-satellite DNA, we have PCR amplified, cloned, and characterized alpha-satellite sequences from the orangutan genome. Nucleotide sequence analysis demonstrated that orangutan alpha-satellite is formed by a single monomeric type that is significantly closer in percentage of sequence identity (mean = 92%, range = 89-96%) to the overall consensus of human alpha-satellite than to the monomeric types corresponding to the four SFs. Use of cloned sequences as hybridization probes to orangutan genomic DNA digested with a panel of restriction enzymes showed that most orangutan alpha-satellite subsets are characterized by a monomeric construction. The subset homologous to clone PPY2-5 is organized in distinct higher-order repeat structures consisting of 18 adjacent monomers. By FISH two clones, PPY3-4 and PPY3-5, proved to be specific for the alpha-satellite on the orangutan homologs of human Chromosomes (Chrs) 10 and 8, respectively. Our data indicate that there was an ancestral monomeric type displaying high sequence similarity to the overall human consensus from which the different great ape and human subsets and SFs may have originated.