Regulation of mouse DNA topoisomerase IIIalpha gene expression by YY1 and USF transcription factors

DNA topoisomerase IIIβ promotes cyst generation by inducing cyst wall protein gene expression in Giardia lamblia

Giardia lamblia causes waterborne diarrhoea by transmission of infective cysts. Three cyst wall proteins are highly expressed in a concerted manner during encystation of trophozoites into cysts. However, their gene regulatory mechanism is still largely unknown. DNA topoisomerases control topological homeostasis of genomic DNA during replication, transcription and chromosome segregation. They are involved in a variety of cellular processes including cell cycle, cell proliferation and differentiation, so they may be valuable drug targets. Giardia lamblia possesses a type IA DNA topoisomerase (TOP3β) with similarity to the mammalian topoisomerase IIIβ. We found that TOP3β was upregulated during encystation and it possessed DNA-binding and cleavage activity. TOP3β can bind to the cwp promoters in vivo using norfloxacin-mediated topoisomerase immunoprecipitation assays. We also found TOP3β can interact with MYB2, a transcription factor involved in the coordinate expression of cwp1-3 genes during encystation. Interestingly, overexpression of TOP3β increased expression of cwp1-3 and myb2 genes and cyst formation. Microarray analysis confirmed upregulation of cwp1-3 and myb2 genes by TOP3β. Mutation of the catalytically important Tyr residue, deletion of C-terminal zinc ribbon domain or further deletion of partial catalytic core domain reduced the levels of cleavage activity, cwp1-3 and myb2 gene expression, and cyst formation. Interestingly, some of these mutant proteins were mis-localized to cytoplasm. Using a CRISPR/Cas9 system for targeted disruption of top3β gene, we found a significant decrease in cwp1-3 and myb2 gene expression and cyst number. Our results suggest that TOP3β may be functionally conserved, and involved in inducing Giardia cyst formation.

Transcriptional analysis of the two reverse gyrase encoding genes of Sulfolobus solfataricus P2 in relation to the growth phases and temperature conditions

Sulfolobus solfataricus, a hyperthermophilic crenarchaeon, contains two genes encoding reverse gyrases, topR1 and topR2. The steady-state level of their transcripts were quantified during the growth phases for cells maintained either at 72, or 80 degrees C, and after temperature changes from one to the other temperature. The transcripts of both genes are weakly expressed, but the highest level is observed in actively dividing cells, and is almost undetectable in cells in decline phase. During the temperature shift experiments, there is no significant topR2 variation. By contrast, there is a maximum 2.4-fold increase in topR1 transcripts within 30 min after the downshift. After 1 h, the transcript level reaches the level characteristic of cells adapted to the new temperature. After an upward shift, the topR1 expression pattern is inversely regulated with a transient decrease with the same time course. The topR1 expression profile is completely different from that of topR2 after temperature shift experiments; this suggests a different regulation process for the two reverse gyrase genes. The fine tuning of the topR1 transcript expression within a short interval of time after a temperature shift illustrates a rapid adaptation response to temperature change.

Identification of the functional elements in the promoter region of human DNA topoisomerase IIIbeta gene

In this study, we have isolated and characterized the promoter region of the human DNA topoisomerase IIIbeta (hTOP3beta) gene. The 5' RACE assay showed a short exon 1 encoding only the 35-bp untranslated region and suggested the presence of multiple transcription initiation sites. The hTOP3beta gene promoter lacks a canonical TATA box or initiation element and is moderately high in GC content. Transient expression of a luciferase reporter gene under the control of serially deleted 5'-flanking sequence identified an activator element between -141 and -119 upstream of the transcription initiation site and a second regulatory element between -91 and -71. On the basis of scanning mutations of triple nucleotides, we demonstrated that a 5'GGAACC3' element between -117 and -112 plays a critical role in the up-regulation of the basal transcription activity. Changing the 5'GGAACC3' sequence leads to markedly reduced promoter activity. Gel mobility shift assays revealed that the 5'GGAACC3' element is required for DNA binding by the transcription factor complex. These observations lead to the conclusion that the positive regulatory region including the 5'GGAACC3' core element is essential for efficient expression of the hTOP3beta gene as well as for the binding of as yet unidentified regulatory factor(s).

Genomic structure and analysis of transcriptional regulation of the mouse zinc-fingers and homeoboxes 1 (ZHX1) gene

The mouse zinc-fingers and homeoboxes 1 (ZHX1) gene was cloned and its transcriptional regulatory mechanism analysed. The mouse ZHX1 gene spans approximately 29 kb and consists of five exons. Exons 1-3 contain the nucleotide sequence of the 5'-noncoding region of mouse ZHX1 cDNA, exon 4 contains a part of the 5'-noncoding region, an entire coding sequence, and a part of the 3'-noncoding sequence, and exon 5 contains the resulting 3'-noncoding sequence. The ZHX1 gene exists as one copy in the haploid mouse genome. Two species of ZHX1 mRNA with or without the nucleotide sequence of the third exon are produced by an alternative splicing. To investigate the regulatory elements involved in the transcription of the ZHX1 gene, transient DNA transfection experiments with ZHX1/firefly luciferase reporter genes were performed using a lipofection method. Functional analyses of a series of 5'- and 3'-deletion constructs of the reporter genes revealed that the nucleotide sequence between -59 and +50 is required for full promoter activity in mouse embryonal carcinoma F9 cells. Two positive regulatory cis-acting elements in the region were identified. These elements, designated as Box A and Box B, are located between nucleotides -47 and -42 and +22 and +27, respectively, and synergistically stimulate transcription of the mouse ZHX1 gene. Electrophoretic mobility shift assays with specific competitors and antibodies show that PEA3 and Yin and Yang 1 (YY1) bind to Box A and Box B, respectively. Thus, we conclude that PEA3 and YY1 synergistically stimulate the transcription of the ZHX1 gene.

Cell type-dependent regulation of human DNA topoisomerase III alpha gene expression by upstream stimulatory factor 2

Here, we report that an E-box element located within the human topoisomerase III alpha (hTOP3 alpha) gene promoter acts as a cell type-specific enhancer. The upstream stimulatory factor (USF) was shown to specifically recognize the mutationally sensitive E-box element. When assayed by transient transfection with hTOP3 alpha promoter-dependent reporter genes, USF is transcriptionally active in HeLa cells but lacks transcriptional activity in Saos-2 cells. The hTOP3 alpha mRNA level in Saos-2 cells was reduced to about 30% of the level observed for HeLa cells, suggesting that the inactivity of USF in hTOP3 alpha promoter activity may be the cause of the marked reduction of hTOP3 alpha mRNA levels in Saos-2 cells. Using transient transfection assays in HeLa cells, we demonstrated that ectopically expressed USF2, but not USF1, was capable of activating hTOP3 alpha transcription through the E-box element. However, USF2 did not stimulate hTOP3 alpha promoter activity in Saos-2 cells. This cell type-specific regulation of promoter activity by USF2 may provide a mechanism for the differential expression of hTOP3 alpha in various tissues and during developmental stages.