The genomic structure of the human SPEC1 gene reveals complex splicing andclose promoter proximity to the AF1q translocation gene

SPECs are small Cdc42 signaling molecules. In mammals, two genes, SPEC1 and SPEC2, encode proteins of 79 and 84 amino acid residues, respectively. Her e we report the expression and genomic organization of the human SPEC1 gene . Using Northern blot analysis, three major SPEC1 mRNA transcripts of 1.6, 3.3, and 6.3 kb were detected. Identification and sequencing of different s ized SPEC1 cDNA clones revealed that the transcript size heterogeneity was due to alternative splicing in the 3 ' -untranslated region. In addition, a distinct SPEC1 splice variant from within the coding sequence, SPEC1-beta, was identified and detected in a variety of human tissues. Analysis of the genomic organization of SPEC1 revealed that the coding sequence of the SPE C1 isoform. was derived from exons 2, 3 and 4, while the SPEC1-beta isoform was derived from exon 2 and a read-through event of intron 2. Examination of the 5 ' -end of the SPEC1 genomic sequence revealed that AF1q, a previou sly identified gene involved in translocations with the MLL (mixed-lineage leukemia) gene, was 631 bp away in a head-to-head orientation. This interge nic sequence containing the putative promoter region for both SPEC1 and AF1 q genes did not contain a TATA box or CAAT box. Transfection experiments us ing an AF1q promoter luciferase. reporter construct in a variety of cells i ncluding Cos1 cells, Jurkat T-cells, MCF-7 breast cancer cells, and NIH-3T3 fibroblasts showed no promoter activity. In contrast, a SPEC1 promoter luc iferase reporter construct showed high levels of reporter activity in Cos1 and MCF-7 cells, low activity in NIH-3T3 fibroblasts and no activity in Jur kat T-cells. These promoter analyses suggest that although SPEC1 and AF1q g enes share the same promoter region, they are not coordinately regulated. ( C) 2001 Elsevier Science B.V. All rights reserved.