PubMed · 4138207
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Z Zborzil. 1974-10-14. [Studies on Bleomycin].. https://pubmed.ncbi.nlm.nih.gov/4138207/
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The RecBCD enzyme of Escherichia coli promotes recombination preferentially at chi nucleotide sequences and has in vivo helicase and strong duplex DNA exonuclease (exoV) activities. The enzyme without the RecD subunit, as in a recD null mutant, promotes recombination efficiently but independently of chi and has no nucleolytic activity. Employing phage lambda red gam crosses, phage T4 2- survival measurements, and exoV assays, it is shown that E. coli cells in which RecBCD has extensive opportunity to interact with linear chi-containing DNA (produced by rolling circle replication of a plasmid with chi or by bleomycin-induced fragmentation of the cellular chromosome) acquire the phenotype of a recD mutant and maintain this for approximately 2 h. It is concluded that RecBCD is converted into RecBC during interaction with chi by irreversible inactivation of RecD. After conversion, the enzyme is released and initiates recombination on other DNA molecules in a chi-independent fashion. Overexpression of recD+ (from a plasmid) prevented the phenotypic change and providing RecD after the change restored chi-stimulated recombination. The observed recA+ dependence of the downregulation of exoV could explain the previously noted "reckless" DNA degradation of recA mutants. It is proposed that chi sites are regulatory elements for the RecBCD to RecBC switch and thereby function as cis- and trans-acting stimulators of RecBC-dependent recombination.
Although the cyclin-dependent kinase inhibitor p21SDI1 (WAF1/CIP1) has been proposed as the mediator of p53-induced cell cycle arrest following DNA damage, several stimuli now appear to induce SDI1 independent of p53 function. We have examined the behavior of p53 and SDI1 in an isogeneic model by manipulating p53 status in normal diploid human fibroblasts using an amphotropic retroviral vector. Following DNA strand break damage induced by bleomycin, both SDI1 induction and G1-S cell cycle arrest are p53 dependent, consistent with SDI1 being the key mediator. In contrast, in cellular senescence (and following UV irradiation), induction of SDI1 occurs independent of p53 function yet growth arrest is still p53 dependent. We conclude (a) that redundant pathways exist for induction of SDI1, but that (b) SDI1, while perhaps necessary, is not sufficient for inhibition of cell cycle progression, requiring the cooperation of an additional factor (possibly another cyclin-dependent kinase inhibitor) whose expression, at least in the case of senescence, is strictly p53 dependent.
Individuals at increased risk for cancer development have been reported to exhibit increased mutagen sensitivity as detected by the G2 phase chromosome breakage assay. To better understand the basis for such chromosome sensitivity, we examined 4 lymphoblastoid cell lines derived from 4 head and neck cancer patients, 2 of whom were previously shown to have normal bleomycin sensitivity and 2 high bleomycin sensitivity (and multiple primary tumors). These cell lines were studied for cell survival and initial damage and repair at both the DNA and chromosome levels, and the results compared to a normal control cell line and 3 ataxia telangiectasia homozygote cell lines. While all cell lines exhibited nearly equal levels of initial DNA damage and repair throughout the cell cycle, both the ataxia telangiectasia homozygote cell lines and 2 cell lines derived from individuals with multiple head and neck primary tumors showed increased levels of initial chromosome damage (detected by premature chromosome condensation), a reduced fast repair component and higher residual chromosome damage. Our results suggest that one component of the enhanced mutagen sensitivity phenotype observed in cancer-prone individuals may involve an inherent chromatin alteration that allows a more efficient translation of DNA damage into chromosome damage following mutagen exposure. The degree of such an alteration might then be associated with an increased risk for second primary tumors in other carcinogen-exposed sites.