[Analysis of the rat novel major histocompatibility complex class II genes, RT1.Ha, RT1.Hb and RT1.DOa]

Rat major histocompatibility complex (RT1) class II genes consist of RT1.D, B and recently defined H. These genes are equivalent to human HLA-DR, HLA-DQ and HLA-DP, respectively. Up to date, RT1.D and B genes have been identified and their nucleotide sequences established. I have cloned the RT1.Ha, Hb and a novel class II a gene which is homologous to human HLA-DNA and named as DOa. The Hb is evidently a pseudogene since it has a 7 bp deletion in the beta 2 domain which cause a frameshift mutation. On the other hand, the Ha gene is intact and its mRNA is expressed at a very low level in the spleen, since spliced mRNA fragment is detectable by reverse transcription and polymerase chain reaction (RT-PCR) but not by conventional Northern analysis. The DNA sequence of DOa is intact and expressed at a moderate level compared with other class II genes. These findings suggest that RT1.DOa product may be functional and may associate with beta chain protein, the product of RT1.DOb gene (formerly RT1.Bb2, although only the sequence coding for beta 2 domain has been reported). Finally, by using an Ha fragment as a probe, Southern analysis detected a DNA fragment hybridizing with it in the genome of mouse. This finding suggests the presence of, at least a fragment of, H-2Pa gene in the genome of the mouse.