Brief Det~nitive Report Intragenic Recombination in an Ea Gene for a Murine Ia Antigen*

T he plasma membranelocated murine Ia antigens consist of two polymorphic chains: the larger chain, designated a, is ~35,000 mol wt, whereas the smaller chain, designated fi, is ~28,000 mol wt (1). Whereas the I-A antigens are encoded by genes (Aa and AO) located within the I-A subregion, the I-E antigens are encoded by one gene (E~) located in the I-E subregion and a second (Ep or A,) 1 in the I-A subregion (2). Previous studies from our laboratories have used F1 hybrids to explore the serological (3, 4) and biochemical (5) basis for the complementa t ion between the E~ and E# genes. These studies, which established that complementa t ion could occur between the genes involved in the trans configuration, provided evidence that the structural genes for the a and fl chains were the complement ing genetic elements. Dur ing the course of the biochemical studies (5), we compared the Ep chain derived from a (D2.GD × A.TFR5)Fa (A a, E b × A t, Ek) 2 to the E~ chain from B10.D2. Al though the EO chain from the F1 animals (presumably encoded by the D2.GD parental chromosome in the F1) was quite similar to the E~ chain from B10.D2, they were not identical. This result was not expected because D2.GD (H-2 ~a) is a recombinant strain previously typed as K d Ad[Bb J b E b C b S b D b, and therefore it should possess the same E~ gene as B 10.D2 (H-2d). This difference between the E a chain encoded by the D2.GD genome and other bona fide E~ chains was also noted by Jones (6), who used two-dimensional (2D) electrophoresis to document the difference. Two obvious and mutual ly exclusive hypotheses may be put forward to explain the aberrant nature of the E~ chain from D2.GD. The first of these is that the recombinational event in D2.GD occurred not between the I-A and the I-B subregions, as