The three murine histocompatibility loci, K, D, 
and Z, collectively make up the H-2 region of the 
murine MHC. The H-2 genes are members of the 
much larger class I multigene family of the MHC 
(~30 genes), which also includes the Qa and Tla 
region genes. Alleles of each H-2 locus exhibit high 
sequence variation (diversity) and are very polymor- 
phic in the population. Alleles of Qa and Tla region 
genes exhibit very little sequence diversity and are 
much less polymorphic than H-2 loci. Although the 
Qa and Tla region genes and their products share 
sequence homology and biochemical characteris- 
tics with the H2 region genes and molecules, their 
biological role is unknown. 
Genetic Recombination in Murine Cells 
Mutant A* genes have been detected in C57BL/6 
mice (by skin grafting) at a high frequency of ~ 1 
per 2,500. (Alleles of H-2 loci from the C57BL/6 
strain are identified by a & superscript.) Sequence 
analyses have indicated that these mutant iiC* genes 
contain clustered, multiple nucleotide alterations. 
The finding that other class I genes contain the iden- 
tical sequences as substituted into the mutant 
gene provides evidence that the mutant genes are 
generated by a recombination event between the A* 
gene and other class I genes. 
These recombinations, which result in the 
transfer of very small segments of DNA (<100 nu- 
cleotides) from donor genes to K'', have been 
termed microrecombinations. The microrecombi- 
nant K'' genes are repetitive in nature; that is, the 
identical microrecombinant genes have been de- 
tected in independently arising mutant mice. It is 
thought that the products of many microrecombina- 
tion events, accumulating in the genes of a mouse 
population, result in the high sequence diversity 
observed among X* alleles. 
The emphasis of the research in Dr. Geliebter's 
laboratory is the analysis of the parameters and re- 
quirements of the microrecombination process. To 
gain further insight into this process, the laboratory 
is using in vitro engineered constructs to detect mi- 
crorecombinant H-2 genes. 
Dr. Geliebter's associates have constructed a fu- 
sion gene in which /3-galactosidase sequences re- 
place two cytoplasmic exons of the X* gene. The 
fusion protein can be detected by staining for 
i8-galactosidase activity, which is manifested as blue- 
colored cells. They have also site-directed two in- 
frame termination codons in the ^ gene at posi- 
tions that undergo frequent microrecombinations. 
This prevents the expression of /3-galactosidase. 
/3-Galactosidase expression can be rescued by a mi- 
crorecombination with a linked class I gene, Q4, 
which recombines away the termination codons. 
Thus microrecombinations can be scored as blue 
cells. 
This microrecombination construct, once intro- 
duced into a variety of cell types, will help deter- 
mine the microrecombination frequencies of differ- 
ent cells. Its introduction into transgenic mice will 
help determine microrecombination frequencies in 
germ cells. Data from previous studies indicate that 
microrecombinations occur in female germ cells. 
The laboratory also hopes to determine if microre- 
combinations occur in sperm cells as well, and at 
what frequency. 
Some strains of mice may undergo microrecom- 
binations more frequently than others. Placing this 
construct onto different genetic backgrounds may 
help to determine microrecombination frequencies 
in different mouse strains, and perhaps identify criti- 
cal parameters in the microrecombination process. 
These studies will contribute to an understanding of 
the genetic processes that control the evolution and 
ultimately the function of the mammalian immune 
system. 
Identification of a Diverse Qa Gene 
While sequence diversity and polymorphism are 
the hallmark of H-2 genes, Qa and Tla region genes 
are characterized by sequence conservation among 
alleles and limited polymorphism. In fact, the lack 
of polymorphism among these genes has been sug- 
gested to preclude an immunological function for 
their products. Dr. Geliebter's laboratory has identi- 
fied a Qa region gene whose sequence differs 
greatly between alleles of the C57BL/6 and C3H 
mice. The sequence diversity between the two al- 
leles is manifested in both scattered and clustered 
nucleotide substitutions. The clustered substitu- 
tions are similar to those observed in the microre- 
combinations that diversify^ H-2 genes, and may re- 
flect past microrecombination events with H-2 and 
other Qa region genes. These data may provide the 
first evidence that Qa genes can be recipients in the 
microrecombination process. 
PCR (polymerase chain reaction) analysis has in- 
dicated that this Qa gene is transcribed in the thy- 
mus of at least some strains of mice. Dr. Geliebter's 
laboratory is currently engaged in an in-depth analy- 
sis of the transcription, translation, and cell surface 
expression of this gene and its product in order to 
ascertain its function. The gene is polymorphic in at 
least three strains of mice, and the analysis of other 
strains is under way. The diversity and polymor- 
phism of the gene suggest an immunological func- 
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