Summary'Abbreviation used in this paper. UTR, untranslated region . In the mouse and human, mRNA transcripts encoding the lymphocyte-specific protein tyrosine kinase p56ikk are derived from two separate promoters resulting in heterogeneity in the 5' untranslated region sequence. The proximal promoter lies just 5' to the coding region for the gene and is active only in thymocytes . In contrast, the distal promoter lies 34 kilobases (kb) 5' in the human, and is active both in thymocytes and mature peripheral T cells . As previously reported, transgenic mice bearing functional proximal promoter sequence juxtaposed with the SV40 large T antigen gene invariably develop lymphoid tumors confined to the thymus. In the current work, transgenic mice bearing a 2 .6-kb fragment of the human distal promoter fused to the SV40 large T antigen gene express large T antigen in thymocytes and in peripheral lymphoid cells, and develop tumors of both the thymus and the peripheral lymphoid organs . The ability of the human distal promoter to function appropriately in transgenic mice is consistent with the strong similarity observed between the mouse and human distal promoter sequences . With the exception of a single short interval that serves as a target for binding of nuclear factors, significant sequence similarity is not seen when the distal and proximal promoter sequences are compared. Hence, developmentally regulated, lineage-specific transcription of the Ick gene is mediated by distinct promoter sequences that appear to be capable of functioning independently.

The lck gene encodes a lymphocyte-specific protein-tyrosine kinase that is implicated in neoplastic transformation. We have determined the germ line organization of the murine lck gene and have isolated and characterized a rearranged lck allele in the murine lymphoma cell line LSTRA. The overall exon-intron organization of the normal lck gene is almost identical to that of avian c-src. In LSTRA DNA, an internally rearranged Moloney murine leukemia virus genome is interposed between two distinct promoters that normally generate lck transcripts differing only in 5' untranslated regions. The rearrangement appears to have been selected to permit splicing of transcripts that initiate from the Moloney virus promoter to an acceptor site located within the first exon 3' to the downstream promoter, thus generating an lck mRNA with a novel 5' untranslated region that may be more efficiently translated.

The lck gene encodes a membrane-associated protein tyrosine kinase that is expressed specifically in lymphoid cells, especially thymocytes. Structural analysis of the murine and human lck genes previously identified conserved 5' flanking sequences that were proposed to represent transcriptional regulatory elements. Here we demonstrate that a murine lck promoter construct containing these sequences directs the expression of the SV40 T-antigen gene in lymphoid cells. Remarkably, expression of SV40 T-antigen in transgenic animals dramatically disturbs thymic development, resulting in preferential loss of CD4+CD8+ thymocytes. In contrast, immature cells lacking both CD4 and CD8 markers are present in near-normal numbers. Thus SV40 T-antigen expression appears partially to arrest thymopoiesis. Mice bearing the lck-SV40 transgene develop readily explantable thymic tumors at 12-18 weeks of age. Fluorocytometric analyses of lck-SV40 tumor cells reveal that immature thymocytes are frequently immortalized. The lck-SV40 mouse may therefore provide materials for the in vitro investigation of thymocyte differentiation.

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