The common ancestry of genes and spacers in the euchromatic region:iomnis ordinis hereditarium a ordinis priscum minutum/i

摘要

Genes in the euchromatic region of mammalian chromosomes can be compared to oases in a barren stretch of desert. The average distance between neighboring genes is estimated to be 35 kbp (kilo-base pairs). This great preponderance of intergenic spacers in the euchromatic region is due mostly to the extreme inefficacy of the mechanism of gene duplication as a means of creating new genes with altered active sites. For every redundant copy of the pre-existent gene that emerged triumphant as a new gene, hundreds of other copies must have degenerated to join the rank of junk DNA. The average functional half-life of redundant gene copies has been estimated as 50 million years. Not surprisingly, each still functioning gene of mammals is, as a rule, flanked by copies in varying degrees of degeneracy, so-called silent pseudogenes. I propose that the entire euchromatic region, genes and spacers included, evolved from a few primordial sequences. The primordial sequence was (AGCTG)(AGCTG)(AGCTG) (GGGTG) in the case of the regions housing the immunoglobulin family of genes. Deep in the interior of each long intergenic spacer dwells a core made of exact replicas of this primordial sequence, and each core is flanked by its degenerate subfamilies of repeats. Between the nearest core and either end, 2 to 3 kbp away, of each long spacer, subfamilies of repeats are arranged in a descending order of relatedness to the primordial sequence as well as in their intra-subfamilial sequence fidelity among copies. Consequently, sequence repetitiousness becomes cryptic in the terminal region of the spacer immediately upstream or downstream of a gene. Nevertheless, coding sequences of the gene are contiguous with adjacent regions of the spacer in every sense, for the coding sequence also embodies the cryptic repetitiousness within it. The contiguity is made evident by the indiscriminate occurrence of descendants of the same building block in the coding sequence and adjacent spacers. It is the choice of its particular primordial building block ultimately derived from (AGCTG)(AGCTG)(AGCTG)(GGGTG) which gives the specific characteristics to each family of genes. For example, the ancestral gene for immunoglobulin light chain constant regions of both x and X classes apparently arose from about 20 tandem repeats of the 36-base-long primordial building block: CCA-TCT-GTC-GTG-AAG-CTG-CTA-AAG-AAC-ATC-AGT-CGC.