Fig. 1

Transposon domestication and co-option of a DNA motif originated the Bex/Tceal gene family. a Diagram showing the domestication of L1 fragments to originate the TCEAL7 gene ORF and 3′UTR. The target site duplication (TSD) and the inversion junction (Inv Jun) resulting from the transposition process [19] are shown. The identity (ID) value was obtained by aligning the transposon nucleotide sequences with the corresponding region of the human TCEAL7 gene. L1MEe and L1ME4a-derived sequences of TCEAL7 are drawn as belonging to the same L1ME-like retrotransposon. The protein domains of the ORF1p of HAL1b are indicated: CC, coiled coil; RRM, RNA-recognition motif; and CTD, carboxy-terminal domain. b Protein alignment of the HAL1b ORF1p and the N-terminus of TCEAL7 proteins from three placental species: Hsa, Homo sapiens; Eca, Equus caballus; Laf, Loxodonta africana. c Diagram showing the proposed evolutionary scenario for the formation of the BGW cluster. (1) A proto-BGW motif lay upstream of the alpha-galactosidase (Gla) promoter (P α) in the X chromosome of the ancestor of eutherians and metatherians. (2) In the eutherian lineage, a Hnrnph1 transcript was retrotranscribed and inserted upstream of Gla and next to the ancestral BGW motif. (3) The co-option of this BGW motif (P BGW) gave rise to Hnrnph2 retrogene. This genomic region was duplicated, and retrotransposons HAL1b and L1ME-like inserted nearby. (4) The new retrotransposon-derived ORF fell under the transcriptional influence of a BGW motif (P′ BGW). The YY1 binding site derived from the 5′UTR of HAL1b [20] was preserved. (5) The BGW motif (P″ BGW) and the YY1 binding site of a Bex/Tceal gene duplicated upstream of a retrocopy of the Armc10 gene, giving rise to the ArmcX ancestral gene. (6) Before the diversification of placentals, the Bex/Tceal and ArmcX gene families expanded forming the BGW cluster