Evidence that a human β-tubulin pseudogene is derived from its corresponding mRNA

Abstract

Pseudogenes—sequences which are homologous to functional genes but which contain mutational changes precluding the formation of a functional product—seem to be a common feature of eukaryotic genomes. Such sequences were first described in the 5S gene of Xenopus laevis¹ and have since been found in several gene families including those of the α- and β-globins of several species^2–10, immunoglobulin V_κ genes¹¹, the actin genes of Dictyostelium¹² and the small nuclear RNA genes of man¹³. Among the globin pseudogenes, there appear to be two kinds of structural organization: genes that have retained their intervening sequences^4–7 and those that have lost them completely^2,3. A possible explanation for the generation of such intron-lacking genes involves the reverse transcription of a processed mRNA to form cDNA, and the introduction of this cDNA copy into the genome either by recombinant heteroduplex formation², insertion into a staggered chromosomal break³, or via a retrovirus intermediate¹⁴. Here we report the complete sequence of a human β-tubulin pseudogene. The sequence data reveal the absence of any intervening sequences, and the presence, in the genome, of an uninterrupted 17 base-pair (bp) tract of A residues 14 base-pairs 3′ to a poly(A) signal (AATAAA)¹⁵. This sequence organization corresponds closely to the sequence organization of the poly(A) signal and poly (A) tract in a β-tubulin mRNA¹⁶.