The human immunoglobulin x locus. Characterization of the duplicated O regions

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Abstract
Two large regions of the human immunoglobulin x locus, the so-called O regions, have been characterized on cosmid and phage λ clones. The two regions are very similar but not identical duplicates belonging to the Cx proximal (p) and the distal (d) copies of the x locus. The Op and Od regions comprise contigs of 90 and 120 kb, respectively, and contain 20 Vx genes and pseudogenes which have been sequenced. Three pairs of Vx genes were found to be practically identical in the duplicates while allotypic differences, at least for two of the genes, are considerable. The similarities between the duplicate genes may be related to the fact that the two copies of the x locus are arranged in a palindrome-like fashion with the 5′ sides of the O regions pointing towards each other (CxJx B Lp Ap Op-Od Ad Ld). This may have contributed to equalizing the sequences. Beyond Op and Od no further Vx genes were found within about 80 kb. Instead, repetitive DNA sequences have been localized there, the structures of which suggest that they may have been involved in the evolution of the Vx gene-containing regions. The Vx pseudogene containing W regions, that had been transposed in evolution from the short to the long arm of chromosome 2 by a pericentric inversion, may have been derived from the O regions according to structural homologies between defined sections of the O and W regions.