Extension of the H-2 TLb molecular map

A region of the TL ^b locus encompassing T11 to T13 contains retroviral sequences TLev1 and TLev2. As part of a study to determine whether the retroviral elements are involved in the expression of TL genes, the genomic organization of this region was reexamined in greater detail. A result of these investigations is the extension of the H-2 TL^b molecular map. Two additional TL genes have been isolated from C57BL/6 mice, T14 and T15. The genomic organization of T9 through T15 is presented. The nucleotide sequence has been determined for exons 4, 5, and 6 of T13. As a result of a C to T conversion, a termination codon is introduced into exon 4, indicating that T13 either encodes a secreted protein or is a pseudogene. T13 was found to be more homologous to the H-2 genes outside the TL region. T14 has been physically disrupted by the integration of TLevl , and the H-2 sequences appear to have diverged greatly. The relationship of the TL regions of the b and c haplotypes has been investigated using numerous low copy probes. The genome of BALB/c (TL^c) is shown to lack a counterpart of the T13–T15 ^b region. Homologous regions exist in the two haplotypes; yet considerable polymorphism is observed. TL^b mice do not express TLa on the cell surface of normal thymocytes while TL^c mice do; TLa expression is activated in many TO leukemias. The diversity seen in the T13–T15 region may provide insights into the phenotypic expression or regulatory mechanisms of TL expression in these two haplotypes.