Dag E. Helland

Properties and mechanism of action of eukaryotic 3-methyladenine-DNA glycosylases.

SUMMARY 3-Methyladenine-DNA glycosylase activities have been identified in all eukaryotic cell systems studied. Some of the results from these studies are reviewed here. The enzymes possess molecular weights between 24×103 and 34×103, they have a broad pH optimum at approximately pH8, require double-stranded DNA and act in the absence of any cofactors. The enzyme can excise several different methylated bases from DNA such as 3-methyladenine, 7-methylguanine and 3-methylguanine. The specific activity of this DNA glycosylase in mouse L-cells was found to be a function of the proliferative state of the cell. In vitro quantification of this DNA repair activity in synchronized mouse L-cells suggests that it is regulated within a defined temporal sequence prior to the onset of DNA replication. Using DNA fragments of defined sequences it was observed that the efficiency of removal of the methylated bases is sequence-dependent.

gp160 of HIV-I synthesized by persistently infected Molt-3 cells Is terminally glycosylated: Evidence that cleavage of gp160 occurs subsequent to oligosaccharide processing

The envelope glycoprotein of HIV-I in infected, cultured human T cells is synthesized as a precursor of apparent Mr 160 kDa (gp160) and is cleaved to two glycoproteins, gp120 and gp41, which are the mature envelope glycoproteins in the virus. Neither the temporal and spatial features of glycosylation nor the oligosaccharide processing and proteolytic cleavage of the envelope glycoprotein are well understood. To understand more about these events, we investigated the glycosylation and cleavage of the envelope glycoproteins in the CD4+ human cell line, Molt-3, persistently infected with HIV-I (HTLV IIIB). The carbohydrate analysis of gp160 and gp120 and the behavior of the glycoproteins and glycopeptides derived from them on immobilized lectins demonstrate that both of these glycoproteins contain complex- and high-mannose-type Asn-linked oligosaccharides. In addition, the N-glycanase-resistant oligosaccharides of gp120 were found to contain N-acetyl-galactosamine, a common constituent of Ser/Thr-linked oligosaccharides. Pulse-chase analysis of the conversion of [35S]cysteine-labeled gp160 showed that in Molt-3 cells it takes about 2 h for gp120 to arise with a half-time of conversion of about 5 h. At its earliest detectable occurrence, gp120 was found to contain complex-type Asn-linked oligosaccharides. Taken together, these results indicate that proteolytic cleavage of gp160 to gp120 and gp41 occurs either within the trans-Golgi or in a distal compartment.

Wavelength dependence for human redoxy-endonuclease-mediated DNA cleavage at sites of UV-induced photoproducts.

Redoxy-endonuclease, an enzyme present in human and other cells, recognizes monobasic photoproducts that occur primarily at sites of cytosine following UV-irradiation of DNA at 254 nm. The wavelength dependence for formation of these photoproducts was determined using end-labeled DNA fragments of defined sequence irradiated with monochromatic light ranging from 254-360 nm as substrates for redoxy-endonuclease partially purified from HeLa cells. The base specificity and extent of DNA cleavage were determined by analysis of the enzyme-generated DNA scission products on DNA sequencing gels. Maximal incision at sites of cytosine and thymine was observed at 280 nm, suggesting that these photoproducts may be relevant with respect to the biological effects of solar radiation.

Systemic and mucosal antibody responses to group B streptococci following immunization of the colonic-rectal mucosa

The cervico-vaginal mucosa is poorly designed for inducing a mucosal immune response, but it can effect such a response evoked at other mucosal sites. This study was undertaken to determine whether colonic-rectal immunization with group B streptococci (GBS) might induce a local cervico-vaginal immune response. Mice were immunized with either fragmented GBS rectally, whole GBS rectally, or whole GBS subcutaneously. Cholera toxin (CT) was used as an adjuvant for the rectal immunizations. Following colonic-rectal immunization with whole GBS, the mean anti-GBS IgA antibody level in vaginal secretions was 735 kU/ml, with individual values reaching 3480 kU/ml. Corresponding levels of IgA antibodies never exceeded 10 kU/ml in serum and intestinal secretions, or 90 kU/g in feces. In vaginal secretions IgA antibodies to GBS also constituted a much larger fraction of total IgA than in serum, intestinal secretions and feces. Immunizations with fragmented GBS produced much lower IgA responses. Anti-GBS IgA response at the inductive site in the colon-rectum was not significant, as opposed to a strong anti-CT IgA response. Except in serum, the anti-GBS IgG responses to colonic-rectal immunizations were generally low, or absent. The results may provide a basis for the development of mucosal vaccines against GBS-infection.

Separation of damage specific DNA endonuclease activities present in calf thymus

A DNA endonuclease activity present in calf thymus specific for incision on DNA damaged by ultraviolet light, osmium tetroxide, potassium permanganate, hydrogen peroxide and acid has been purified from whole cell extracts. The enzymatic activity was heterogeneous both with regard to molecular mass and charge. The molecular mass of the enzyme varied from 25 to 35 kDa, but the different enzymatic species appeared to possess similar activities. The enzymes acted equally well on damage in supercoiled and relaxed forms of DNA. It further had a narrow optimum with regard to salt concentrations, the optimum activity being observed at a concentration of KCl from 40 to 65 mM.

Chromosomal assignment of human uracil-DNA glycosylase to chromosome 12.

Using Southern blot analysis of DNA from a panel of rodent-human somatic cell hybrids with known karyotypes, we have assigned the human uracil-DNA glycosylase gene to chromosome 12.

Characterization of HIV-1 reverse transcriptase with antibodies indicates conformational differences between the RNAse H domains of p 66 and p 15

SummaryAntibody binding to the p66 and p 15 RNase H regions of HIV-1 reverse transcriptase was compared using a polyclonal rabbit immune serum raised against a synthetic peptide from the RNase H region of reverse transcriptase (aa 511–527) and six monoclonal antibodies binding to discontinuous epitopes in the RNase H region of p66. The antigens used in Western blot analysis included recombinantly expressed homodimeric p 66 digested with the HIV-1 protease for generation of the p 51 and p 15 polypeptides and two different length RNase H domains expressed as Trp E fusion proteins (aa410–560 and aa 441–560). The polyclonal rabbit antibody binding to a continuous epitope recognized both the Trp E-fusion proteins and also the polypeptides p 66 and p 15 generated by processing of homodimeric p66 with the viral protease. Two additional cleavage products with estimated molecular weights of 9 and 11 kDa were also detected. The anti-RNase H MAbs binding to discontinuous epitopes recognized only the RNase H domain of the p 66 polypeptide and the Trp E-RNase H fusion protein when this was expressed together with the C-terminal part of the polymerase domain. The results indicate conformational differences between the RNase H domain of the p66 subunit and the RNase H p 15 polypeptide.

Structural and functional similarities between HIV-1 reverse transcriptase and the Escherichia coli RNA polymerase β′ subunit

Four monoclonal antibodies (MAbs) recognizing HIV‐1 reverse transcriptase (RT) were shown here to cross‐react with the β′ subunit of Escherichia coli RNA polymerase (RNAP). The anti‐RT MAbs bind to a peptide comprising residues 294–305 of the RT amino acid sequence. Computer analyses revealed sequence similarity between this peptide and two regions of the RNAP β′ subunit. MAb‐binding studies using RT mutants suggested that the epitope is located to amino acids 652–663 of the β′ sequence. One of the MAbs which inhibited the polymerase activity of RT also mediated a dose dependent inhibition of the RNAP activity.

Damage Specific Mammalian Endonucleases

A broad spectrum of agents are known to react with DNA, and the resulting changes in structure may have deleterious biological consequences, if not removed. Modified bases in DNA can be excised from DNA by different repair pathways. The process of excision repair involves four major steps: incision, excision, DNA synthesis, and ligation of the repaired sequence. The enzymes responsible for the first step in this scheme are damage-specific enzymes of which several have been isolated and characterized both from bacteria and eukaryotic cells. This group of enzymes is known to recognize damage in double-stranded DNA induced by a variety of agents, such as UV- and gamma-radiation, acid, psoralen, and alkylating agents.

Complementary transcribed T4 RNA. Association with the polysomes

Abstract Pulse labelled bacteriophage T 4 RNA isolated from polysomes from either early or late infected cells were found to contain complementary RNA. The fraction of complementary late RNA was higher in the heaviest late polysomes. The RNA not associated with polysomes appeared to contain insignificant amounts of complementary RNA. The significance of these findings are discussed.