CHARACTERIZATION OF THE MURINE B7-1 GENOMIC LOCUS REVEALS AN ADDITIONAL EXON ENCODING AN ALTERNATIVE CYTOPLASMIC DOMAIN AND A CHROMOSOMAL LOCATION OF CHROMOSOME-16, BAND B5

The murine B7-1 (mB7-1) molecule expressed on APCs delivers a costimul atory signal for T cell activation through its T cell counter-receptor CD28, resulting in T cell proliferation and IL-2 production. Signalin g through the TCR in the absence of CD28 signaling results in T cell a nergy. We have analyzed the genomic structure of mB7-1 and here descri be the identification of a previously unrecognized sixth exon at the f ar 3' end of the locus, which encodes an alternative cytoplasmic domai n. Reverse transcriptase-PCR amplification of mB7-1 transcripts demons trates that exon 6 is functionally spliced to the transmembrane-encodi ng exon 4. Furthermore, using 5' rapid amplification of cDNA ends, we determined that the 5'-untranslated region extends over 1505 bp beyond the previously reported transcriptional start site. In addition, we r eport the chromosomal location of mB7-1 to chromosome 16, band B5.