A novel putative transcription factor protein MYT2 that preferentially binds supercoiled DNA and induces DNA synthesis in quiescent cells

Interaction in vitro of type III intermediate filament proteins with supercoiled plasmid DNA and modulation of eukaryotic DNA topoisomerase I and II activities

To further characterize the interaction of cytoplasmic intermediate filament (cIF) proteins with supercoiled (sc)DNA, and to support their potential function as complementary nuclear matrix proteins, the type III IF proteins vimentin, glial fibrillary acidic protein, and desmin were analyzed for their capacities to interact with supercoiled plasmids containing a bent mouse gamma-satellite insert or inserts capable of non-B-DNA transitions into triplex, Z, and cruciform DNA, that is, DNA conformations typically bound by nuclear matrices. While agarose gel electrophoresis revealed a rough correlation between the superhelical density of the plasmids and their affinity for cIF proteins as well as cIF protein-mediated protection of the plasmid inserts from S1 nucleolytic cleavage, electron microscopy disclosed binding of the cIF proteins to DNA strand crossovers in the plasmids, in accordance with their potential to interact with both negatively and positively supercoiled DNA. In addition, the three cIF proteins were analyzed for their effects on eukaryotic DNA topoisomerases I and II. Possibly because cIF proteins interact with the same plectonemic and paranemic scDNA conformations also recognized by topoisomerases, but select the major groove of DNA for binding in contrast to topoisomerases that insert into the minor groove, the cIF proteins were able to stimulate the enzymes in their supercoil-relaxing activity on both negatively and positively supercoiled plasmids. The stimulatory effect was considerably stronger on topoisomerase I than on topoisomerase II. Moreover, cIF proteins assisted topoisomerases I and II in overwinding plasmid DNA with the formation of positive supercoils. Results obtained with the N-terminal head domain of vimentin harboring the DNA binding region and terminally truncated vimentin proteins indicated the involvement of both protein-DNA and protein-protein interactions in these activities. Based on these observations, it seems conceivable that cIF proteins participate in the control of the steady-state level of DNA superhelicity in the interphase nucleus in conjunction with such topoisomerase-controlled processes as DNA replication, transcription, recombination, maintenance of genome stability, and chromosome condensation and segregation.

DNA secondary structure and Raman markers of supercoiling in Escherichia coli plasmid pUC19

Negative supercoiling in the 2686 bp Escherichia coli plasmid pUC19 is comparable in linking number (Lk(0) = 258) and superhelical density (sigma = -0.05) to the moderate supercoiling exhibited by many eukaryotic chromosomal DNAs in vivo. Supercoiled and relaxed forms of purified pUC19 in aqueous solution (0.1 M NaCl, pH 8.3, 20 degrees C) have been investigated by Raman spectroscopy to assess changes in B-DNA secondary structure induced by superhelical stress and to identify putative Raman markers of DNA supercoiling. We find that supercoiling leads to small but significant changes to the B-form Raman signature of linear DNA. Spectral band shifts in the 780-850 cm(-1) interval are interpreted as resulting from a small net change in the average phosphodiester torsions alpha (O3'-P->-O5'-C5') and zeta (C3'-O3'->-P-O5') from the gauche(-)/gauche(-) range to the gauche(-)/trans range with supercoiling. The magnitude of the spectral intensity change implies that approximately 5% of the nucleotide moieties are affected. Supercoiling also introduces small redistributions of Raman intensity within the 1460-1490 and 1660-1670 cm(-1) intervals, consistent with small structural perturbations. Importantly, no Raman markers of Watson-Crick base pairing, base stacking, or C2'-endo/anti deoxynucleoside conformations are perturbed significantly by supercoiling of pUC19, indicating that the B-DNA structure is largely conserved under moderate superhelical stress. Peak and trough features at 814 and 783 cm(-1), and at 1462 and 1489 cm(-1), respectively, in the Raman difference spectrum between superhelical and relaxed DNA are proposed as markers of moderate negative supercoiling. We also show that in Tris-buffered solutions the Raman signature of supercoiled DNA can be obscured by Raman bands of Tris counterions. The subtle structural perturbations to B-DNA induced by moderate supercoiling are consistent with proposed mechanisms of transcriptional activation.