Antibodies against DNA-psoralen crosslink recognize unique conformation

Mitochondrial DNA replication proceeds via a 'bootlace' mechanism involving the incorporation of processed transcripts

The observation that long tracts of RNA are associated with replicating molecules of mitochondrial DNA (mtDNA) suggests that the mitochondrial genome of mammals is copied by an unorthodox mechanism. Here we show that these RNA-containing species are present in living cells and tissue, based on interstrand cross-linking. Using DNA synthesis in organello, we demonstrate that isolated mitochondria incorporate radiolabeled RNA precursors, as well as DNA precursors, into replicating DNA molecules. RNA-containing replication intermediates are chased into mature mtDNA, to which they are thus in precursor-product relationship. While a DNA chain terminator rapidly blocks the labeling of mitochondrial replication intermediates, an RNA chain terminator does not. Furthermore, processed L-strand transcripts can be recovered from gel-extracted mtDNA replication intermediates. Therefore, instead of concurrent DNA and RNA synthesis, respectively, on the leading and lagging strands, preformed processed RNA is incorporated as a provisional lagging strand during mtDNA replication. These findings indicate that RITOLS is a physiological mechanism of mtDNA replication, and that it involves a 'bootlace' mechanism, in which processed transcripts are successively hybridized to the lagging-strand template, as the replication fork advances.

Native DNA fragments photocrosslinked to psoralen binds to anti-B and anti-Z DNA antibodies

Calf thymus DNA fragments of varying size (100-600 bp) were covalently linked to psoralen in the presence of 365 nm UV light. The crosslinked species were separated from the monoadducts by hydroxyapatite column chromatography. The DNA-psoralen crosslink showed binding with naturally occurring anti-DNA autoantibodies. This was substantiated by the recognition of affinity purified IgG by DNA-psoralen crosslinks of varying DNA size. The photo-crosslinks exhibited considerable recognition of both monoclonal and polyclonal anti-Z-DNA antibodies in inhibition ELISA and band shift assay.

Cross-reactions of human lupus autoantibodies with 8-methoxypsoralen photomodified DNA fragments

DNA fragments of around 200 base pair (average size) have been covalently crosslinked with 8-methoxypsoralen under 365 nm UV light. The photoadduct, induced antibodies in rabbits with a titer of > 1:12,800 by direct binding ELISA. Binding data showed that the induced antibodies are conformation-specific recognizing restricted conformational change at site of crosslinking. Human autoantibodies against DNA, bound not only to native DNA but to the photomodified DNA fragment as well. In addition, binding patterns of SLE sera obtained from different patients were remarkably similar, indicating the recognition of altered conformation of the modified polymer by naturally occurring SLE anti-DNA autoantibodies.

Autoantibodies-like antigen binding characteristics of induced antibodies against polylysine-polyglutamate complex

Rabbits immunized with polylysine-polyglutamate complex induced high titer antibodies. The immune IgG exhibited high degree of specificity towards the immunogen. In a competition assay, nDNA, heat denatured DNA and total buffalo thymus RNA were inhibitory. An appreciable binding with polyglutamate, poly(rG).poly(dC) poly(dG).poly(dC), cardiolipin, poly(dA-dU). Poly(dA-dU) and Z- or Z-like conformations demonstrates the polyspecificity of induced antibodies. The immunological specificity of induced antibodies was comparable to autoantibodies derived from SLE. The results might be of some help in understanding the etiology of autoimmune process.

Immunological probing of induction and repair of 8-methoxypsoralen photoadducts in DNA from Fanconi anemia and normal human fibroblasts: quantitative analysis by electron microscopy

A direct method of immuno-electron microscopy has been employed to simultaneously determine 8-methoxypsoralen (8-MOP) photoinduced monoadducts (MA) and interstrand cross-links (CL), their relative localization along the DNA molecule, and their removal. It has been applied to DNA from cultures of Fanconi anemia (FA) fibroblasts (complementation groups A and D), and of normal human fibroblasts, following treatment by 8-MOP and 365 nm radiation. The immuno-reaction with monoclonal antibody 8G1 was performed on DNA extracted from the cells just after photoreaction, or after a 24 h repair period, and then denatured. Furan-side MA and also a significant proportion of pyrone-side MA were very efficiently immuno-detected. Only 1-2% CL were IgG-labeled. This is why CL were directly visualized and quantified on denatured DNA from the same cellular samples as used for immuno-detection. Results demonstrate that FA group A cells are not only impaired in the incision of CL, but also of MA. The response of FA group D cells is intermediate between normal and FA group A cells.

Antibodies against free radical modified native DNA recognize B-conformation

Hydroxyl radical, a prominent entity of reactive oxygen species, is known to modify cellular DNA and has been implicated in several human diseases. In the present studies, the radical was generated by exposure of hydrogen peroxide to 254 nm light in the presence of native calf thymus DNA. Single strand breaks, decrease in Tm and modification of adenine and thymine were some of the modifications observed in nDNA. Antibodies induced in experimental animals against the modified DNA were immunogen specific. These antibodies also recognize native B-conformation. It was observed that naturally occurring anti-native DNA autoantibodies from SLE sera recognize modified DNA in direct binding and competition ELISA. Gel retardation assay reiterated the formation of immune complexes between induced antibodies and DNA fragments of around 300 bp (B-conformation). The possible significance of these findings in the etiology of SLE has been discussed.

Naturally occurring SLE anti-DNA antibodies recognize unique conformation on DNA-lysine photoadduct

Native calf thymus DNA has been covalently modified with lysine under UV-A light. Human autoantibodies on purification through affinity column of native DNA linked to polylysyl-Sepharose 4B showed almost equal recognition of DNA and photoadduct. The recognition of DNA-lysine photoadduct by the affinity-purified autoantibodies might be helpful in understanding their origin in SLE vis-à-vis the role of positively charged amino acids in the pathogenesis of autoimmune diseases.