Characterization of the promoter of human leukocyte-specific transcript 1 - A small gene with a complex pattern of alternative transcripts

The gene for the human leukocyte-specific transcript 1 (LST1) encodes a small protein that modulates immune responses and cellular morphogenesis. The LST1 transcripts are expressed at high levels in dendritic cells. Because of the complex splicing pattern, use of alternative 5'-untranslated exons, and a biologically interesting pattern of expression of LST1 mRNA, we studied the human LST1 gene promoter and regulatory elements. We identified an additional upstream 5'-untranslated exon in U937 monocytic cells. Transient transfection studies demonstrated that the combination of regions from -1363 to -621 with -112 to -54, relative to the translation start codon, produced the highest level of transcripts from among the various constructs tested, but the pattern of transcripts produced was only a subset of those produced from the endogenous gene. DNase I footprinting analysis and electrophoretic mobility shift assays showed that oligonucleotide probes corresponding to three regions, -1171 to -1142 (BI), -1136 to -1111 (BII), and -783 to -751 (BTV), bound proteins in U937 nuclear extracts. Competition and supershift electrophoretic mobility shift assay did not identify any known transcription factors responsible for BII probe binding. These studies suggest that a novel DNA-binding site and interaction of multiple regulatory elements may be involved in mediating the expression of the various forms of LST1 mRNA.