Divergent intron arrangement in theMB1/LMP7 proteasome gene pair
We sequenced the humanMB1 gene from a cosmid clone mapping to chromosome 14q11.2–12. The gene spans about 6 kilobases and contains three exons and two introns. There was no evidence of an alternative leader exon, which is a characteristic of the major histocompatibility complex (MHC)-encodedLMP7 gene, the closet relative ofMB1, with which it shares 67% amino acid identity. Conceptual translation of the 5′ end of the gene class for a cleaved leader sequence of 59 amino acids, consistent with western blot data. None of theMB1 gene's three exons were coincident with any of the six exons inLMP7. In contrast, in the delta-encoding gene and its counterpart, the MHC-encodedLMP2 gene (59% amino acid identity), all six exons are arranged at equivalent positions in respect to the coding frame. The unique structure ofMB1 implies a separate origin or different selection pressures acting at this particular locus. DNA repeat analysis provides information on the minimum time of separation of theMB1/LMP7 pair of genes.