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

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Abstract
The central regions of the x locus, the so‐called A regions, have been fully characterized on cosmid and phage λ clones. The regions, which are parts of the Cx‐proximal and‐distal copies of the locus and are, therefore, called Ap and Ad regions, comprise about 140 kb each and contain together 30 Vx genes and pseudogenes. The A regions have been linked on their 5′ sides to the O regions and on their 3′ sides to the L regions. Chromosomal walking has eliminated a previous gap in the Ap region. Detailed restriction maps of the Ap and Ad regions and the sequences of 9 Vx genes are reported. Four events, which have occurred in evolution probably after the duplication of the A region, were identified: the insertion of an Alu element in Ad; the insertion of part of a LINE element in Ap; the deletion of a 17.5‐kb fragment including one Vx gene from Ap; the sequence divergence of duplicated Vx gene regions which ranges among the five pairs studied here from 0 to 14 bp per kb and converted two genes to pseudogenes while their duplicates stayed functional. An analysis of the A regions of the lymphoid cell lines RPMI 6140 and GM607 confirmed the previous finding that the Vx‐Jx rearrangement in these cell lines had occurred by deletion and inversion mechanisms, respectively. Thus, the structural data contribute to the understanding of the evolution and the functioning of the A regions of the x locus.