The gene structure of B-type nuclear lamins of Xenopus laevis: implications for the evolution of the vertebrate lamin family.

The gene structure of the Xenopus laevis lamin B1 gene is presented, together with a partial analysis of the lamin B2 gene of the same species. The intron/exon pattern of lamin B1 is strikingly similar to other vertebrate lamin genes and is completely identical to that of Xenopus lamin B3. An additional intron present in the murine lamin B2 gene has a counterpart in the orthologue gene of Xenopus. These data complete a comparison of the genomic organization of all vertebrate lamin types known so far. They allow the conclusion that the genes encoding B1 and B3 probably reflect the ancestral lamin gene organization and that the B2-type lamins are derived from them. A non-conforming splice junction sequence is found in the Xenopus lamin B1 gene. A GC is found instead of the canonical GT dinucleotide at the 5' end of intron IX. Comparison with other unusual splice sites indicates that the lamin B1 gene represents a functional gene.