RESUMO
Human CC chemokine receptor 5 (CCR5), mediates the activation of cells by the chemokines macrophage inflammatory protein-1alpha, macrophage inflammatory protein-1beta, and RANTES, and serves as a fusion cofactor for macrophage-tropic strains of human immunodeficiency virus type 1. To understand the molecular mechanisms that regulate human CCR5 gene expression, we initiated studies to determine its genomic and mRNA organization. Previous studies have identified a single CCR5 mRNA isoform whose open reading frame is intronless. We now report the following novel findings. 1) Complex alternative splicing and multiple transcription start sites give rise to several distinct CCR5 transcripts that differ in their 5'-untranslated regions (UTR). 2) The gene is organized into four exons and two introns. Exons 2 and 3 are not interrupted by an intron. Exon 4 and portions of exon 3 are shared by all isoforms. Exon 4 contains the open reading frame, 11 nucleotides of the 5'-UTR and the complete 3'-UTR. 3) The transcripts appear to be initiated from two distinct promoters: an upstream promoter (PU), upstream of exon 1, and a downstream promoter (PD), that includes the "intronic" region between exons 1 and 3. 4) PU and PD lacked the canonical TATA or CAAT motifs, and are AT-rich. 5) PD demonstrated strong constitutive promoter activity, whereas PU was a weak promoter in all three leukocyte cell environments tested (THP-1, Jurkat, and K562). 6) We provide evidence for polymorphisms in the noncoding sequences, including the regulatory regions and 5'-UTRs. The structure of CCR5 was strikingly reminiscent of the overall structure of other chemokine/chemoattractant receptors, underscoring an important evolutionarily conserved function for a prototypical gene structure. This is the first description of functional promoters for any CC chemokine receptor gene, and we speculate that the complex pattern of splicing events and dual promoter usage may function as a versatile mechanism to create diversity and flexibility in the regulation of CCR5 expression.
