Alternative sites of transcription initiation upstream of the canonical cap site in human gamma-globin and beta-globin genes

Using S1 mapping and primer extension analysis, we have identified a number of human kappa-globin and beta-globin 5' RNA termini originating in the 200 bp upstream of the canonical mRNA cap sites. Upstream initiation sites have previously been reported for the human epsilon-globin gene (4,5) and the present work indicates that this is a general feature of the human beta-type globin genes. We have attempted to identify features common to such sites between the three genes. One site 170 bp upstream of the major beta-globin cap site and a site 1400 bp upstream of the major epsilon-globin cap site are located near putative PolIII promoter sequences and may therefore be transcribed by this enzyme. Alternative initiation sites located 200 bp and 50-100 bp upstream of the epsilon-globin and kappa-globin cap sites respectively are located within S1 hypersensitive regions of chromatin.     

Nuclease hypersensitivity in the beta-globin gene region of K562 cells

We have investigated chromatin structure in the beta-globin gene region of the K562 human erythroleukemic cell line by using S1 and DNase I nuclease sensitivity assays. Despite the lack of beta-globin gene expression in these cells, we find nuclease-hypersensitive sites to these enzymes in its 5' and 3' flanking regions in K562 chromatin. This result is in contrast to previous reports in which no hypersensitive sites were found in the immediate vicinity of this gene. In the 3' region, one major hypersensitive site at 0.9 kpb 3' and three minor hypersensitive sites at 0.7 kbp, 0.5 kbp 3' and 0.2 kbp 5' of the polyadenylation site were observed; these sites are very similar to those found in fetal liver and adult bone marrow cells in which the beta-globin gene is expressed. We find hypersensitive sites to both enzymes in the 5' region of the beta-globin gene: a major site 0.8 kbp 5' to the cap site, and two minor sites 1.2 and 1.5 kbp 5' to the cap site. The -0.8 kbp site is also present in plasmids containing the beta-globin gene. Our results suggest that the lack of beta-globin gene expression may be related to the lack of hypersensitivity sites in the immediate (150 bp) 5' flanking region of the beta-globin gene, as occurs in other active globin genes.     

Repression of CD2 gene expression is mediated by an AP-2 related factor

Tissue specific and developmental expression of the CD2 gene is tightly regulated during T cell development. DNase I hypersensitivity analysis has revealed the presence of two sites (DHS1 and 2) located 5' to the CD2 gene which have been reported to be implicated in the developmental regulation of expression of CD2. The location of DHS2 marks the position of the minimal promoter whereas DHS1 is located approximately 1800 bp upstream. We show that repressor and derepressor activities are contained within the region of DNA marked by DHS1. The repressor is capable of regulating homologous and heterologous promoters regardless of orientation. This activity is entirely dependent upon the presence of an AP-2 binding site as mutation of this site resulted in a loss of repressor activity. A nuclear factor found in Jurkat cells specifically binds this site but was shown to be serologically distinct from AP-2.