T-CELL RECEPTOR V-ALPHA GENE SEGMENT WITH ALTERNATE SPLICING IN THE JUNCTIONAL REGION

The locus encoding mouse TCR-alpha chain includes approximately 100 Va lpha gene segments that can be organized in about 20 structural subfam ilies. Southern blot analysis of a T cell line derived from the BALB/c strain, M5T, has indicated that both alpha loci were rearranged, as a ssessed by the deletion of the delta locus, and that the Valpha gene s egment involved in one of the rearrangements did not belong to any of the Valpha subfamilies already described. Transcripts of TCR-alpha cha ins from the M5T line were cloned after cDNA synthesis and anchored-po lymerase chain reaction, revealing a Valpha gene segment of an as yet unidentified subfamily, Valpha5T. Molecular cloning of germ-line Valph a5T gene segments has shown that this subfamily contained two members, one of them being a pseudogene. The two members were located to each extremity of the alpha locus associated with a member of the Valpha13 and ValphaBWB subfamilies. Analysis of transcripts bearing the Valpha5 T gene segment in the M5T line as well as in thymocytes has revealed t hat Jalpha are frequently absent. This is due to an alternate donor sp lice site generated at the Valpha5T-Jalpha junction that leads to a sp licing from the end of Valpha5T to Calpha instead of the Jalpha to Cal pha conventional splicing. The impact of Jalpha spliced-out transcript s on the allelic exclusion process is discussed.