Gene structure and regulation

The lymphotoxin genes map within the MHC in humans on chromosome 6 and in the mouse on chromosome 17. They map between the complement (class III) genes and class I MHC genes in close proximity to the TNFa locus (Figure 4). The protein products of these LT/TNF genes share 50% identity at the amino acid level and have common structural motifs. These observations suggest that LTa, LT(3 and TNFa have arisen through a process of gene duplication.

Expression of LT is regulated at several levels. The promoter and 5' negative and positive regulatory elements of the LTa gene have been identified. Positive transcriptional elements include an NFkB element. LTa expression is also regulated at the post-transcriptional level. A sequence located in the 3'

untranslated sequences of the gene, TTATTTAT, has been implicated in mRNA destabilization. LTa mRNA accumulates to high levels in cells in the presence of cycloheximide, suggesting the existence of a labile repressor of mRNA, perhaps a ribonuclease. LTa has a conventional signal peptide. In contrast to TNFa, which can be displayed as a membrane-bound molecule and then released, LTa is transported through the cell and secreted or stored in granules in cytolytic T cells, or forms the LTa/(3 complex as noted above. The promoter region of LT(3 shares several features in common with the LTa promoter, including the presence of a NFi<B-binding site, indicating that LTa and LT(3 expression may be induced by common signals. However, the LT|3 promoter also contains transcription factor-binding sites that are not contained in LTa, implying that the two genes can also be regulated independently. The lack of lymphoid organs in the LTa-deficient mice indicates that this gene is expressed during development.

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