Role of yeast Rth1 nuclease and its homologs in mutation avoidance, DNA repair, and DNA replication

The RTH1(RAD27) gene of Saccharomyces cerevisiae encodes a structure-specific endonuclease that cleaves 5'-ended single-stranded DNA at its junction with duplex DNA. Genetic and biochemical studies have indicated a role of Rth1 nuclease in the removal of RNA primers formed during DNA replication. The rth1Delta mutation confers temperature-sensitive lethality, and increases sensitivity to alkylating agents. The instability of repetitive DNA is greatly enhanced in the rth1Delta mutant. The conditional lethality of the rth1Delta mutation indicates that another nuclease can function in DNA replication in the absence of RTH1. RAD2, a homolog of RTH1, is required for nucleotide-excision repair. Here, we examine three other homologs of RTH1/RAD2 - YEN1, EXO1, and DIN7. Deletion of any of these genes in the rth1Delta strain has no effect on cell viability, suggesting the involvement of another, and as yet unidentified, nuclease in the maturation of Okazaki fragments. Our data also indicate that only RTH1 functions in the repair of alkylation damage. Deletions of YEN1, EXO1, DIN7, or RAD2, either singly or when combined with one another and with the rth1Delta mutation, have no effect on the rate of instability of dinucleotide repeats or on the rate of formation of large duplications in the CAN1 gene. These data provide evidence of a high degree of specificity for the role of RTH1 in DNA replication and in base-excision repair, and for the requirement of RAD2 in nucleotide-excision repair. The possibility that both Rth1 and Exo1 function in DNA mismatch repair is discussed.

Therapeutic uses of microencapsulated genetically engineered cells

Microencapsulated genetically engineered cells have the potential to treat a wide range of diseases. For example, in experimental animals, implanted microencapsulated cells have been used to secrete growth hormone to treat dwarfism, neurotrophic factors for amyotrophic lateral sclerosis, beta-endorphin to decrease pain, factor XI for hemophilia B, and nerve growth factors to protect axotomized neurons. For some applications, microencapsulated cells can even be given orally. They can be engineered to remove unwanted molecules from the body as they travel through the intestine, and are finally excreted in the stool without being retained in the body. This application has enormous potential for the removal of urea in kidney failure, ammonia in liver failure and amino acids such as phenylalanine in phenylketonuria and other inborn errors of metabolism.

ATP-dependent assembly of a ternary complex consisting of a DNA mismatch and the yeast MSH2-MSH6 and MLH1-PMS1 protein complexes

MSH2 and MSH6 proteins exist as a stable complex, as do the MLH1 and PMS1 proteins. To study the mismatch binding properties of the MSH2-MSH6 complex and to examine its functional interaction with the MLH1-PMS1 complex, these protein complexes were purified to near homogeneity from overproducing yeast strains. As has been reported previously, the purified MSH2-MSH6 complex binds DNA substrates containing a G/T mismatch and insertion/deletion mismatches, but the binding affinity for the latter decreases as the size of the extrahelical loop increases. Addition of ATP or the nonhydrolyzable ATPgammaS reduces binding of the MSH2-MSH6 complex to the DNA substrates markedly. Here, we show that MSH2-MSH6 forms a ternary complex with MLH1-PMS1 on a mismatch containing DNA substrate. The formation of this ternary complex requires ATP, which can be substituted by ATPgammaS, suggesting that ATP binding alone is sufficient for ternary complex formation. Thus, it appears that ATP binding by the MSH2-MSH6 complex induces a conformation that is conducive for the interaction with MLH1-PMS1 complex, leading to the formation of the ternary complex.

The DNA-dependent ATPase activity of yeast nucleotide excision repair factor 4 and its role in DNA damage recognition

Saccharomyces cerevisiae RAD7 and RAD16 genes function together in the nucleotide excision repair of transcriptionally inactive DNA. The RAD7- and RAD16-encoded proteins exist as a tight complex named nucleotide excision repair factor 4 or NEF4. Previously, we showed that NEF4 binds UV-damaged DNA with high specificity and with a dependence upon ATP and that inclusion of NEF4 to the reconstituted nucleotide excision repair system consisting of purified NEF1, NEF2, NEF3, and replication protein A results in marked stimulation of damage-specific DNA incision. Here we show that NEF4 possesses an ATPase activity that is entirely dependent on a DNA cofactor and that double-stranded DNA is twice as effective as single-stranded DNA in activating ATP hydrolysis. Even though DNA binding is promoted by the nonhydrolyzable ATP analogue adenosine 5'-O-(thiotriphosphate) (ATPgammaS), damage binding is more proficient with ATP than with ATPgammaS. Interestingly, UV irradiation of double-stranded DNA results in a pronounced attenuation of the ATPase activity. Taken together, our results suggest a model in which ATP hydrolysis by NEF4 fuels the translocation of NEF4 on DNA in search of UV lesions and damage binding by NEF4 leads to a down-regulation of the ATPase activity. Damage-bound NEF4 could then serve as a nucleation point for the assembly of other repair components.

Crystal structure of the Saccharomyces cerevisiae ubiquitin-conjugating enzyme Rad6 at 2.6 A resolution

The Saccharomyces cerevisiae ubiquitin-conjugating enzyme (UBC) Rad6 is required for several functions, including the repair of UV damaged DNA, damage-induced mutagenesis, sporulation, and the degradation of cellular proteins that possess destabilizing N-terminal residues. Rad6 mediates its role in N-end rule-dependent protein degradation via interaction with the ubiquitin-protein ligase Ubr1 and in DNA repair via interactions with the DNA binding protein Rad18. We report here the crystal structure of Rad6 refined at 2.6 A resolution to an R factor of 21.3%. The protein adopts an alpha/beta fold that is very similar to other UBC structures. An apparent difference at the functionally important first helix, however, has prompted a reassessment of previously reported structures. The active site cysteine lies in a cleft formed by a coil region that includes the 310 helix and a loop that is in different conformations for the three molecules in the asymmetric unit. Residues important for Rad6 interaction with Ubr1 and Rad18 are on the opposite side of the structure from the active site, indicating that this part of the UBC surface participates in protein-protein interactions that define Rad6 substrate specificity.

Tandem subdiaphragmatic and pleural sequelae due to lost gallstones following cholecystectomy

We report two similar thoracoabdominal complications we encountered due to retained gallstones after cholecystectomy. These patients had had an open cholecystectomy after a failed laparoscopic attempt, with spillage of gallbladder debris intraoperatively. They were admitted more than 12 months later with subdiaphragmatic abscesses. Attempted computerized axial tomography (CT) guided drainage of these abscesses resulted in these patients developing pleural fluid collections, which required surgical drainage. The patients underwent exploratory laparotomies, and drainage of the subdiaphragmatic abscesses had revealed gallstones within the abscess cavity. A detailed presentation of these cases, with review of current literature and clinicopathologic issues for discussion are described.

Growth and survival of renal failure rats that received oral microencapsulated genetically engineered E. coli DH5 cells for urea removal

In our earlier reports we have introduced a new method for urea removal in renal failure. This is based on the oral administration of polymeric artificial cells containing nonpathogenic genetically engineered E. coli DH5 cells to renal failure rats. This resulted in the lowering of systemic uremic urea level to normal range without elevation of ammonia. The present article deals with the safety study of this approach. Microscopic study follows changes in microcapsule morphology with time are described. Two parameters, the body weight of the renal failure rats and, the survival for of the renal failure rats receiving microcapsules containing genetically engineered E. coli DH5 cells are discussed in this article. Result shows that there is no significant difference between the weight profile of uremic rats receiving oral therapy and normal control rats. Also the treated group of uremic rats survived longer than the untreated uremic rats.

Cloning and characterization of a novel rho-type GTPase-activating protein gene (ARHGAP6) from the critical region for microphthalmia with linear skin defects

Microphthalmia with linear skin defects syndrome (MLS) is an X-linked dominant, male-lethal disorder associated with chromosomal rearrangements that result in deletions of the distal short arm of the X chromosome. In an effort to isolate expressed sequences from the 500-kb MLS critical region in Xp22.3, exons were trapped from 14 overlapping cosmids. Using exon connection followed by cDNA library screening, we identified a 2.4-kb contig of cDNA library screening 170 kb of genomic sequence in the MLS deletion region. Northern analysis of this cDNA detected a prominent approximately 4.2-kb transcript and a less abundant approximately 6-kb transcript in all tissues examined, with additional transcripts in skeletal muscle. Sequence analysis revealed a coding region of 601 amino acids contained in 12 exons, with a splice variant isoform of 495 amino acids. The predicted protein sequence of the gene, named ARHGAP6, contains homology to the GTPase-activating (GAP) domain of the rhoGAP family of proteins, which has been implicated in the regulation of actin polymerization at the plasma membrane in several cellular processes. The possible role of the ARHGAP6 protein in the pathogenesis of MLS is discussed.

Requirement of yeast DNA polymerase delta in post-replicational repair of UV-damaged DNA

DNA lesions in the template strand pose a block to the replication machinery. Replication across such lesions may occur by a mutagenic bypass process in which a wrong base is inserted opposite the lesion or may involve processes that are relatively error-free. Genetic studies in the yeast Saccharomyces cerevisiae have indicated the requirement of REV3-encoded DNA polymerase in mutagenic bypass. The DNA polymerase responsible for error-free bypass, however, has not been identified, but genetic studies implicating proliferating cell nuclear antigen in this process have suggested that either DNA polymerase delta or DNA polymerase epsilon may be involved. Here, we use temperature-sensitive (ts) conditional lethal mutations of the S. cerevisiae POL2 and POL3 genes, which encode DNA polymerase epsilon and delta, respectively, and show that post-replicational bypass of UV-damaged DNA is severely inhibited in the pol3-3 mutant at the restrictive temperature. By contrast, the pol-2-18 mutation has no adverse effect on this process at the restrictive temperature. From these observations, we infer a requirement of DNA polymerase delta in post-replicative bypass of UV-damaged DNA.

Enhancement of MSH2-MSH3-mediated mismatch recognition by the yeast MLH1-PMS1 complex

DNA mismatch repair has a key role in maintaining genomic stability. Defects in mismatch repair cause elevated spontaneous mutation rates and increased instability of simple repetitive sequences, while mutations in human mismatch repair genes result in hereditary nonpolyposis colorectal cancers. Mismatch recognition represents the first critical step of mismatch repair. Genetic and biochemical studies in yeast and humans have indicated a requirement for MSH2-MSH3 and MSH2-MSH6 heterodimers in mismatch recognition. These complexes have, to some extent, overlapping mismatch binding specificities. MLH1 and PMS1 are the other essential components of mismatch repair, but how they function in this process is not known. We have purified the yeast MLH1-PMS1 heterodimer to near homogeneity, and examined its effect on MSH2-MSH3 binding to DNA mismatches. By itself, the MLH1-PMS1 complex shows no affinity for mismatched DNA, but it greatly enhances the mismatch binding ability of MSH2-MSH3.

Yeast DNA repair proteins Rad6 and Rad18 form a heterodimer that has ubiquitin conjugating, DNA binding, and ATP hydrolytic activities

The RAD6 and RAD18 genes of Saccharomyces cerevisiae are required for postreplicative bypass of ultraviolet (UV)-damaged DNA and for UV mutagenesis. The RAD6 encoded protein is a ubiquitin conjugating enzyme, and RAD18 encodes a protein containing a RING finger motif and a nucleotide binding motif. Rad18 can be co-immunoprecipitated with Rad6, indicating that the two proteins exist in a complex in vivo. Here, we co-overproduce the two proteins using a yeast multicopy plasmid, purify the Rad6-Rad18 complex to near homogeneity, and show that the complex is heterodimeric. The Rad6-Rad18 heterodimer has ubiquitin conjugating activity, binds single-stranded DNA, and possesses single-stranded DNA-dependent ATPase activity. The Rad6-Rad18 complex provides the first example wherein a ubiquitin conjugating activity is physically associated with DNA binding and ATPase activities provided by an associated protein factor. The co-existence of these activities should provide the complex with the ability to recognize single-stranded DNA resulting from stalling of the replication machinery at DNA damage sites and to recognize the components of the DNA replication machinery for ubiquitination by Rad6.

Yeast Rad7-Rad16 complex, specific for the nucleotide excision repair of the nontranscribed DNA strand, is an ATP-dependent DNA damage sensor

In eukaryotes, nucleotide excision repair of ultraviolet light-damaged DNA is a highly intricate process that requires a large number of evolutionarily conserved protein factors. Genetic studies in the yeast Saccharomyces cerevisiae have indicated a specific role of the RAD7 and RAD16 genes in the repair of transcriptionally inactive DNA. Here we show that the RAD7- and RAD16-encoded products exist as a complex of 1:1 stoichiometry, exhibiting an apparent dissociation constant (Kd) of <4 x 10(-10) M. The Rad7-Rad16 complex has been purified to near homogeneity in this study and is shown to bind, in an ATP-dependent manner and with high specificity, to DNA damaged by ultraviolet light. Importantly, inclusion of the Rad7-Rad16 complex in the in vitro nucleotide excision repair system that consists entirely of purified components results in a marked stimulation of damage specific incision. Thus, Rad7-Rad16 complex is the ATP-dependent DNA damage sensor that specifically functions with the ensemble of nucleotide excision repair factor (NEF) 1, NEF2, NEF3, and replication protein A in the repair of transcriptionally inactive DNA. We name this novel complex of Rad7 and Rad16 proteins NEF4.

Domains required for dimerization of yeast Rad6 ubiquitin-conjugating enzyme and Rad18 DNA binding protein

The RAD6 gene of Saccharomyces cerevisiae encodes a ubiquitin-conjugating enzyme required for postreplicational repair of UV-damaged DNA and for damage-induced mutagenesis. In addition, Rad6 functions in the N end rule pathway of protein degradation. Rad6 mediates its DNA repair role via its association with Rad18, whose DNA binding activity may target the Rad6-Rad18 complex to damaged sites in DNA. In its role in N end-dependent protein degradation, Rad6 interacts with the UBR1-encoded ubiquitin protein ligase (E3) enzyme. Previous studies have indicated the involvement of N-terminal and C-terminal regions of Rad6 in interactions with Ubr1. Here, we identify the regions of Rad6 and Rad18 that are involved in the dimerization of these two proteins. We show that a region of 40 amino acids towards the C terminus of Rad18 (residues 371 to 410) is sufficient for interaction with Rad6. This region of Rad18 contains a number of nonpolar residues that have been conserved in helix-loop-helix motifs of other proteins. Our studies indicate the requirement for residues 141 to 149 at the C terminus, and suggest the involvement of residues 10 to 22 at the N terminus of Rad6, in the interaction with Rad18. Each of these regions of Rad6 is indicated to form an amphipathic helix.

Studies on leaching of Cr and Ni from stainless steel utensils in certain acids and in some Indian drinks

Leachates of Cr and Ni from stainless steel utensils viz., frying pans, bowls and tumblers, have been investigated, by exposing the utensils to decinormal solutions of citric, tartaric and lactic acids and to some common Indian drinks. A comparison of observed results indicate that the complexation of metal ions with organic acid anions is most vital and metal leaching is largely a function of the availability of free anions. The intake of Cr and Ni by human beings has also been calculated.

Synthesis of corneal keratan sulfate proteoglycans by bovine keratocytes in vitro

Keratan sulfate proteoglycans (KSPGs) are the major proteoglycans of the cornea and are secreted by keratocytes in the corneal stroma. Previous studies have been able to show only transient secretion of KSPG in cell culture. In this study, cultures of bovine keratocytes were found to secrete the three previously characterized KSPG proteins into culture medium. Reactivity with monoclonal antibody I22 demonstrated substitution of these proteins with keratan sulfate chains. KSPG constituted 15% of the proteoglycan metabolically labeled with [35S]sulfate in keratocyte culture medium. This labeled KSPG contained keratan sulfate chains of 4700 Da compared to 21,000 Da for bovine corneal keratan sulfate. Labeled keratan sulfate from cultures contained nonsulfated, monosulfated, and disulfated disaccharides that were released by digestion with endo-beta-galactosidase or keratanase II. Nonsulfated disaccharides were relatively more abundant in keratan sulfate from culture than in corneal keratan sulfate. These results show that cultured bovine keratocytes maintain the ability to express all three of the known KSPG proteins, modified with keratan sulfate chains and sulfated on both N-acetylglucosamine and galactose moieties. KSPG made in vitro differs from that found in vivo in the length and sulfation of its keratan sulfate chains. The availability of cell cultures secreting corneal keratan sulfate proteoglycans provides an opportunity to examine biosynthesis and control of this important class of molecules.

Dual requirement for the yeast MMS19 gene in DNA repair and RNA polymerase II transcription

Genetic and biochemical studies of Saccharomyces cerevisiae have indicated the involvement of a large number of protein factors in nucleotide excision repair (NER) of UV-damaged DNA. However, how MMS19 affects this process has remained unclear. Here, we report on the isolation of the MMS19 gene and the determination of its role in NER and other cellular processes. Genetic and biochemical evidence indicates that besides its function in NER, MMS19 also affects RNA polymerase II (Pol II) transcription. mms19delta cells do not grow at 37 degrees C, and mutant extract exhibits a thermolabile defect in Pol II transcription. Thus, Mms19 protein resembles TFIIH in that it is required for both transcription and DNA repair. However, addition of purified Mms19 protein does not alleviate the transcriptional defect of the mms19delta extract, nor does it stimulate the incision of UV-damaged DNA reconstituted from purified proteins. Interestingly, addition of purified TFIIH corrects the transcriptional defect of the mms19delta extract. Mms19 is, however, not a component of TFIIH or of Pol II holoenzyme. These and other results suggest that Mms19 affects NER and transcription by influencing the activity of TFIIH as an upstream regulatory element. It is proposed that mutations in the human MMS19 counterpart could result in syndromes in which both NER and transcription are affected.

Assessment of iodine deficiency disorders using the 30 cluster approach in the National Capital Territory of Delhi

OBJECTIVE

Iodine Deficiency Disorders (IDD) are a major public health problem in India. The National Capital Territory (NCT) of Delhi is a known iodine deficiency endemic area. The Delhi Government banned the sale of non-iodised salt since 1989. The present study was conducted to assess the status of IDD after 7 years of salt iodisation programme in the state.

DESIGN

Cross sectional.

METHODOLOGY

The recent indicators recommended by the World Health Organization-United Nations Childrens Fund-International Council for the Control of Iodine Deficiency Disorders (WHO-UNICEF-ICCIDD) were utilized for the assessment of IDD. A total of 30 clusters were selected using population proportionate to size cluster sampling procedure. In each identified cluster, one primary school was selected using random sampling. A total of 6911 school children in the age group of 8-10 years were included for the study.

RESULTS

The total goiter prevalence rate was 8.6% while 2.1, 8.4, 17.6 and 71.9% of the children had urinary iodine excretion levels of < 2, 2-4.9, 5-9.9 and 10 and above mcg/dl, respectively. The median urinary iodine excretion was 17 mcg/dl. Of the 1854 salt samples analyzed, salt with a nil iodine content was consumed only by 1.4% of the beneficiaries. Forty one per cent of families consumed salt with an iodine content of less than 15 ppm.

CONCLUSION

IDD continues to be a public-health problem in the NCT of Delhi. There is a need of strengthening the existing monitoring system for the quality of iodised salt.

Evidence for involvement of yeast proliferating cell nuclear antigen in DNA mismatch repair

DNA mismatch repair plays a key role in the maintenance of genetic fidelity. Mutations in the human mismatch repair genes hMSH2, hMLH1, hPMS1, and hPMS2 are associated with hereditary nonpolyposis colorectal cancer. The proliferating cell nuclear antigen (PCNA) is essential for DNA replication, where it acts as a processivity factor. Here, we identify a point mutation, pol30-104, in the Saccharomyces cerevisiae POL30 gene encoding PCNA that increases the rate of instability of simple repetitive DNA sequences and raises the rate of spontaneous forward mutation. Epistasis analyses with mutations in mismatch repair genes MSH2, MLH1, and PMS1 suggest that the pol30-104 mutation impairs MSH2/MLH1/PMS1-dependent mismatch repair, consistent with the hypothesis that PCNA functions in mismatch repair. MSH2 functions in mismatch repair with either MSH3 or MSH6, and the MSH2-MSH3 and MSH2-MSH6 heterodimers have a role in the recognition of DNA mismatches. Consistent with the genetic data, we find specific interaction of PCNA with the MSH2-MSH3 heterodimer.

Transcription factor TFIIH and DNA endonuclease Rad2 constitute yeast nucleotide excision repair factor 3: implications for nucleotide excision repair and Cockayne syndrome

Nucleotide excision repair (NER) of ultraviolet light-damaged DNA in eukaryotes requires a large number of highly conserved protein factors. Recent studies in yeast have suggested that NER involves the action of distinct protein subassemblies at the damage site rather than the placement there of a "preformed repairosome" containing all the essential NER factors. Neither of the two endonucleases, Rad1-Rad10 and Rad2, required for dual incision, shows any affinity for ultraviolet-damaged DNA. Rad1-Rad10 forms a ternary complex with the DNA damage recognition protein Rad14, providing a means for targeting this nuclease to the damage site. It has remained unclear how the Rad2 nuclease is targeted to the DNA damage site and why mutations in the human RAD2 counterpart, XPG, result in Cockayne syndrome. Here we examine whether Rad2 is part of a higher order subassembly. Interestingly, we find copurification of Rad2 protein with TFIIH, such that TFIIH purified from a strain that overexpresses Rad2 contains a stoichiometric amount of Rad2. By several independent criteria, we establish that Rad2 is tightly associated with TFIIH, exhibiting an apparent dissociation constant < 3.3 x 10(-9) M. These results identify a novel subassembly consisting of TFIIH and Rad2, which we have designated as nucleotide excision repair factor 3. Association with TFIIH provides a means of targeting Rad2 to the damage site, where its endonuclease activity would mediate the 3' incision. Our findings are important for understanding the manner of assembly of the NER machinery and they have implications for Cockayne syndrome.

Requirement of proliferating cell nuclear antigen in RAD6-dependent postreplicational DNA repair

The proliferating cell nuclear antigen (PCNA) acts as a processivity factor for replicative DNA polymerases and is essential for DNA replication. In vitro studies have suggested a role for PCNA-in the repair synthesis step of nucleotide excision repair, and PCNA interacts with the cyclin-dependent kinase inhibitor p21. However, because of the lack of genetic evidence, it is not clear which of the DNA repair processes are in fact affected by PCNA in vivo. Here, we describe a PCNA mutation, pol30-46, that confers ultraviolet (UV) sensitivity but has no effect on growth or cell cycle progression, and the mutant pcna interacts normally with DNA polymerase delta and epsilon. Genetic studies indicate that the pol30-46 mutation is specifically defective in RAD6-dependent postreplicational repair of UV damaged DNA, and this mutation impairs the error-free mode of bypass repair. These results implicate a role for PCNA as an intermediary between DNA replication and postreplicational DNA repair.

Study of mercury-selenium (Hg-Se) interactions and their impact on Hg uptake by the radish (Raphanus sativus) plant

Pot culture experiments were conducted to study the effects of selenite and selenate treatment (0.5-6.0 microg/ml) on the uptake and translocation of root-absorbed mercury (Hg) in radish plants irrigated with 2 and 5 microg/ml Hg in sand and soil culture. Statistically significant reductions in mercury uptake with increasing concentrations of selenium (Se) were observed. Both forms of selenium (selenite and selenate) were equally effective in reducing the mercury burden of the plant. The observed reduction in plant uptake of mercury is explained by the formation of an HgSe insoluble complex in the soil-root environment. No significant difference (P > 0.05) in dry matter yields with the various selenium treatments was found, suggesting that no selenium toxicity or salt injury occurred in the plants.