3 research outputs found
RNA topoisomerase is prevalent in all domains of life and associates with polyribosomes in animals
DNA Topoisomerases are essential to resolve topological problems during DNA metabolism in all species. However, the prevalence and function of RNA topoisomerases remain uncertain. Here, we show that RNA topoisomerase activity is prevalent in Type IA topoisomerases from bacteria, archaea, and eukarya. Moreover, this activity always requires the conserved Type IA core domains and the same catalytic residue used in DNA topoisomerase reaction; however, it does not absolutely require the non-conserved carboxyl-terminal domain (CTD), which is necessary for relaxation reactions of supercoiled DNA. The RNA topoisomerase activity of human Top3β differs from that of Escherichia coli topoisomerase I in that the former but not the latter requires the CTD, indicating that topoisomerases have developed distinct mechanisms during evolution to catalyze RNA topoisomerase reactions. Notably, Top3β proteins from several animals associate with polyribosomes, which are units of mRNA translation, whereas the Top3 homologs from E. coli and yeast lack the association. The Top3β-polyribosome association requires TDRD3, which directly interacts with Top3β and is present in animals but not bacteria or yeast. We propose that RNA topoisomerases arose in the early RNA world, and that they are retained through all domains of DNA-based life, where they mediate mRNA translation as part of polyribosomes in animals
Apoptosis-induced chromosome breaks and chromosome rearrangements at mixed lineage leukaemia breakpoint cluster region in nasopharyngeal and leukaemia cells
Chromosome rearrangements such as additions, deletions, translocations and inversions
are commonly observed in various types of cancer. In leukaemia, the Mixed Lineage
Leukaemia (MLL) gene on chromosome llq23 had been found to translocate with many
partner genes, resulting in the generation of fusion genes. Chromosomal deletions and
additions at llq23 had been reported in nasopharyngeal carcinoma (NPC) though it was
unknown which gene was involved. The apoptotic nuclease was proposed to be involved
in these chromosomal rearrangements by fragmenting the genomic DNA into small
fragments forming the nucleosomal DNA ladder. In leukaemia, it was possible that the
MLL gene and its partner genes were being cleaved during apoptosis. In an effort to
rescue the cells, the DNA repair system may erroneously join the DNA fragments,
resulting in chromosome translocation. Epstein-Barr virus (EBV) infection and/or other
factors may induce apoptosis in NPC cells, resulting in chromosomal breaks and
subsequent rejoining may take place during the attempted DNA repair. As a result, the
surviving cells may harbour chromosome rearrangements such as deletions. Therefore,
the aim of this study was to detect apoptosis-induced chromosome breaks and
rearrangements using NPC and leukaemic cell lines as model systems. These cell lines
were induced to undergo apoptosis by overgrowing the cells to high densities followed by
revival in fresh medium to rescue them. The leukaemic cells were also induced to
undergo apoptosis by treatment for short time period with the low and high
concentrations of etoposide (VP-16). This was followed by revival of the apoptotic cells
in fresh medium to rescue them. In addition, both the NPC and leukaemic cells were
also treated with different concentrations of hydrogen peroxide (H202) to induce
apoptosis. Subsequently, detection of chromosome rearrangements and breaks was performed. Nested PCR using primers specific for MLL and AF9 (one of the common
translocation partners of MLL) was also performed. For NPC cells, nested PCR using
the primers specific for the MLL bcr and the telomere were carried out to detect deleted
chromosome with telomere sequence added. Our result showed that both the cell
density-induced apoptosis in NPC and leukaemic cells generated chromosomal breaks
near the 3' end of the MLL bcr. There was no telomere capturelhealing detected at the
cleavage sites of the revived NPC cells. The sequenced PCR bands of multiple sizes were
found to originate from other chromosomes. Both the 3' and 5' breaks of the MLL bcr
were detected in the revived VP-16-treated leukaemic cells. In some of the samples,
these 5' breaks were located centromeric to the MLL bcr region. However, no
translocation of other genes to the break sites was detected. NPC cells treated with
H202 showed chromosomal breaks near the 3' end of the MLL bcr with no telomere
sequences added. However, in leukaemic cells, translocation of the MLL bcr with
unknown genes from other chromosomes was detected