Rise and fall of the delta globin gene

Abstract

This paper examines the molecular basis for the origin and subsequent silencing of the delta globin gene during the evolution of higher primates. We cloned and sequenced the delta gene from representatives of the two most distantly related types of Old World monkeys in which the gene is silent. The silent delta genes of these two monkeys (rhesus and colobus) are highly homologous to each other (97.5%) and to the functional human delta gene (94%). The presumed mutation that silenced the delta gene is therefore unlikely to be obscured by later mutations at the same site. This silencing mutation could have affected transcription since we find no delta transcript in rhesus bone marrow. Furthermore, the delta globin genes from colobus and rhesus monkeys are many times less efficient than the human delta gene in a cell-free transcription assay. The two Old World monkeys share only three nucleotide substitutions in the first 100 base-pairs of the DNA sequence 5' to the site for initiation of transcription when compared to human delta. It is likely that the in vitro silencing of the delta gene was brought about by one (or a combination) of these three substitutions. Three analysis shows how the delta sequences are related to those for other beta-like genes, including the colobus beta gene whose sequence we report. One part of the delta gene appears to be derived from a (pseudo?) beta gene that diverged from beta before the divergence of mice and men. The other part is closely related to the beta gene of higher primates. The mosaic nature of the delta gene is apparently due to a partial conversion which took place between the pseudo beta and the functional beta gene before the divergence of the lineages leading to Old World monkeys and hominoids.