Identification and characterization of a chicken alpha-globin enhancer.

We identify and describe the properties of an enhancer within the chicken alpha-globin gene cluster. This cluster consists of one gene (pi) expressed only in primitive erythrocytes and two (alpha A and alpha D) expressed in both primitive and definitive cell lineages. The genes are linked together in the order 5'-pi-alpha D-alpha A-3' and occupy a region about 10 kilobase pairs long. The enhancer is located at the 3' end of the cluster, about 750 base pairs 3' to the alpha A translation stop site. When assayed by transfection into either primitive or definitive primary chicken erythrocytes, this element stimulated expression from plasmids containing the alpha D- or alpha A-globulin gene promoters. Except for sites in the alpha-globin promoters, no other stimulatory activity was observed in DNA taken from other regions of the alpha-globin locus. Moderate resolution DNase I hypersensitivity studies as well as DNase I footprinting revealed three regions of protein binding, each containing a similar core DNA sequence within the enhancer element. Gel mobility shift studies demonstrated that all three regions bind the recently identified erythrocyte-specific factor, EryfI, which has binding sites in the regulatory regions of all chicken globin genes. Our data suggest that the enhancer we have identified may act in vivo only on the alpha A gene; expression of the alpha D gene is affected by another EryfI site located in the alpha D promoter. Such a mechanism would be consistent with the observed relative abundances of alpha A- and alpha D-globin in vivo. The simplicity of these regulatory elements may reflect the limited repertoire of expression of these genes during development.     

CCAAT box-dependent activation of the TATA-less human DNA polymerase alpha promoter by the human cytomegalovirus 72-kilodalton major immediate-early protein.

Human cytomegalovirus (HCMV) immediate-early (IE) proteins are known potent transregulators of viral and cellular gene expression upon HCMV infection. HCMV is known to activate a number of cellular genes intimately associated with the cell cycle and DNA replication by mechanisms involving the viral major IE 86-kDa protein (IE2). We have recently shown that IE2 mediates this activation in a TATA-dependent manner and interacts directly with the TATA-binding protein. However, a number of TATA-less cellular promoters, e.g., DNA polymerase alpha and dihydrofolate reductase, are also activated by HCMV infection. Consequently, we have asked how HCMV mediates this activation. We show that, consistent with its known TATA dependency, IE2 does not activate the DNA polymerase alpha promoter. In contrast, this promoter is strongly activated by the major IE 72-kDa protein (IE1). Whilst deletion of ATF or E2F sites within the DNA polymerase alpha promoter had little effect on IE1-mediated activation, removal of the CCAAT box appeared to abolish high levels of activation by IE1. Consistent with this observation, we also find that IE1 interacts directly with the CCAAT box binding factor CTF1 in vitro and massively augments CTF1-mediated activation of the DNA polymerase alpha promoter in transient transfection assays.     

Studies of c-myb gene regulation in MRL-lpr/lpr mice. Identification of a 5' c-myb nuclear protein binding site and high levels of binding factors in nuclear extracts of lpr/lpr lymph node cells

Lymph node T cells of MRL-lpr/lpr mice are characterized by the production of very large amounts of c-myb mRNA. To study the control of c-myb expression, a search was made for sites on the 5' c-myb gene which could bind regulatory proteins. DNase I digestion of nuclear chromatin uncovered four DNase I hypersensitive sites in the first intron of the c-myb gene, and a single site approximately 300 bp 5' to the initiation codon. Lambda exonuclease digestion of a 5'-myb fragment in the presence of nuclear extracts from either MRL-lpr/lpr PLN or EL-4 thymoma revealed stop sites approximately 300 bp 5' (-271 to -322) to the ATG initiation codon. DNase I footprint analysis demonstrated a guanine-cytosine enriched region of potential binding sites (-274 to -319) in the region of the stop sites and a fifth potential binding site closer to the initiation codon (-163 to -168). Specific gel shift bands were detected by a 5'-myb fragment (-346 to -155) with extracts from a number of different lymphoid cell lines and the appropriate specific and non-specific competitor DNA. The DNA giving rise to these gel shift bands encompassed the region defined by the stop site and footprinting studies. To determine whether or not the protein binding to the 5' c-myb gene at -274 to -319 was associated with increased c-myb mRNA, we studied nuclear extracts of several cell lines and compared the amount of binding to the amount of c-myb mRNA found on Northern analyses. Among the cell lines, there was a correlation between c-myb expression and the amount of the 5'-myb DNA binding protein. In addition, MRL-lpr/lpr lymph node cells had high c-myb expression and large amounts of the 5'-myb binding protein. This result suggests that the binding may play some role in the c-myb expression. Moreover, the most immature cell lines had the greatest amount of the binding factor, suggesting that its regulatory effect on c-myb expression might be important in early differentiation events.     

A 10-base-pair element of the human immunodeficiency virus type 1 long terminal repeat (LTR) is an absolute requirement for transactivation by the human cytomegalovirus 72-kilodalton IE1 protein but can be compensated for by other LTR regions in transactivation by the 80-kilodalton IE2 protein.

Transient gene expression studies have indicated that human cytomegalovirus (HCMV) specifically transactivates the human immunodeficiency virus (HIV) long terminal repeat (LTR). We show here, by a specific mutational analysis, that only the TATA box region is obligatory for transactivation of the HIV-1 LTR by HCMV. Similarly, this element is also sufficient for transactivation by either the HCMV 72-kDa major immediate-early 1 (IE1) or 80-kDa IE2 gene product independently. However, deletion of a 10-bp region from the minimal responsive element, 5' to the TATA box, dramatically reduced the level of HCMV 72-kDa IE1 or 80-kDa IE2 transactivation, indicating a crucial role for this element in transactivation. Whereas inclusion of the TAR element or Sp1 sites on this 10-bp-deleted minimal promoter had no effect on the removal of IE1 transactivation, TAR and Sp1 elements did compensate for the 10-bp element in transactivation by IE2 and HCMV. Consequently, the sequence requirements of the HIV-1 LTR for transactivation by HCMV can be reproduced by these IE1 and IE2 gene products of HCMV.