Control of the expression of genes for DNA ligase in eukaryotes

Development of non-electrophoretic assay method for DNA ligases and its application to screening of chemical inhibitors of DNA ligase I

A new rapid assay method for DNA ligases has been developed, which allows direct quantification of enzyme activity without using the traditional polyacrylamide gel electrophoretic technique. In this method, the 5'-biotinylated nicked duplex was used as a substrate for the ligase reaction, in which the 5'-end of the first oligonucleotide (19-mer) on the nicked strand is biotinylated and the second oligonucleotide (20-mer) on the same strand is labeled with radioactive 32P at the 5'-end. After ligation of the biotinylated 19-mer oligonucleotide into the second oligonucleotide with the reaction of DNA ligases, the biotinylated 19-mer oligonucleotide is converted into the radioactive 39-mer oligonucleotide. The ligase reaction products were heat-denatured to release both ligated and unligated biotinylated oligonucleotides. The biotinylated oligonucleotides were then captured on a streptavidin-coated microtiter plate and counted. The results obtained using this method correlated very well with those from the standard assay method using electrophoresis. Using this assay method, we were able to screen a chemical library and identify new DNA ligase inhibitors structurally related to resorcinol, which has growth inhibitory effects on the human breast cancer cell, MCF-7. The method described here is anticipated to be very useful for screening DNA ligase inhibitors from chemical libraries.

Expression of DNA ligases I and II during oogenesis and early development of Xenopus laevis

We have analyzed the expression of DNA ligase I protein during oogenesis and early development of Xenopus laevis. The protein is already present in stage I oocytes and then accumulates throughout oogenesis to reach a steady state level by stage VI. It remains at this level at least until tadpole stage. In stage VI oocytes DNA ligase I protein is almost exclusively localized in the germinal vesicle. We have partially purified a DNA ligase II activity from stage VI oocytes, unfertilized eggs, and stage 8 embryos. An 80-kDa polypeptide can be specifically adenylated in all three purified extracts. It is not recognized by antibodies directed against DNA ligase I and is active on oligo(dT)-poly(rA) substrate. It could therefore represent DNA ligase II protein. The presence of both DNA ligases I and II in oocytes and embryos is inconsistent with the DNA ligase model that had been previously proposed for amphibia.

Identification of a specific inhibitor for DNA ligase I in human cells

A protein inhibitor for human DNA ligase I has recently been identified. It was copurified with a fraction of the enzymes from HeLa cells through several steps of chromatography. The inhibitor was first identified by the absence of ligation activity of the associated enzyme, while it retained the ability to form the ligase-[32P]AMP adducts. The inhibitor was eluted as a single peak at approximately 0.25-0.30 M NaCl from a Mono S column. It inhibited the ligation of both double-stranded and single-stranded breaks by purified DNA ligase I but not by T4 DNA ligase and DNA ligase II. Subsequent gel-filtration chromatography indicated that this inhibitor, with a molecular mass of 55-75 kDa, could form a complex with DNA ligase I and inhibited the DNA ligation activity. Rechromatography of the ligase I-inhibitor complex in high-salt conditions resulted in the dissociation of the complex and the restoration of enzyme activity, indicating that the physical interaction of inhibitor with DNA ligase I is one of the mechanisms of inhibition. These data indicate that this protein inhibitor for DNA ligase I may play a specific role in regulating DNA ligation during replication, repair, or recombination.

Reinvestigation of DNA ligase I in axolotl and Pleurodeles development

We have recently shown that the exclusion process causing the replacement of DNA ligases II by DNA ligase I in amphibian eggs after fertilization does not occur in the case of Xenopus laevis [Hardy, S., Aoufouchi, S., Thiebaud, P., and Prigent, C., (1991) Nucleic Acids Res. 19, 701-705]. Since this result is in contradiction with the situation reported in axolotl and Pleurodeles we decided to reinvestigate such results in both species. Three different approaches have been used: (1) the substrate specificity of DNA ligase I; (2) the DNA ligase-AMP adduct reaction and (3) the immunological detection using antibodies raised against the X.laevis DNA ligase I. Our results clearly demonstrate that DNA ligase I activity is associated with a single polypeptide which is present in oocyte, unfertilized egg and embryo of both amphibians. Therefore, the hypothesis of a change in DNA ligase forms, resulting from an expression of the DNA ligase I gene in axolotl and Pleurodeles early development must be rejected. We also show that, in contradiction with published data, the unfertilized sea urchin egg contains a DNA ligase activity able to join blunt ended DNA molecules.

Evidence for a change in expression of DNA ligase genes in the Pleurodeles waltlii germ line during gonadogenesis

The expression of DNA ligase genes was studied using the nuclear transplantation approach in the germ line of Pleurodeles waltlii (P. waltlii) just before and during gonadogenesis. Germ cell (GC) nuclei were isolated from larvae of P. waltlii and transplanted into unfertilized Ambystoma mexicanum eggs. DNA ligase activity in these eggs was then analyzed after sucrose gradient fractionation. The activity of DNA ligase I (heavy form, 7.5 S) of P. waltlii was present when the transplanted GC nuclei were isolated before the first histological appearance of gonadogenesis. At the beginning of genital ridge formation and thereafter, DNA ligase I activity was replaced by that of DNA ligase II (light form, 7 S). Expression of form I was found to be sensitive to inhibitors of translation and transcription, while that of form II was not. Therefore, the change in DNA ligase activity of the transferred nuclei of P. waltlii germ cells was assumed to be the consequence of a change in gene activity, namely, the repression of the gene encoding DNA ligase I. This change in the gene-regulated state could be linked to protein modifications of the chromatin. These results indicate that, at the beginning of gonadogenesis, germ cells receive information leading to a new state of differentiation.