The antimicrobial activity of various naturally occurring microbicidal peptides was reported to result from their interaction with microbial membrane. In this study, we investigated the cytotoxicity of the hemolytic peptide dermaseptin S4 (DS4) and the nonhemolytic peptide dermaseptin S3 (DS3) toward human erythrocytes infected by the malaria parasite Plasmodium falciparum. Both DS4 and DS3 inhibited the parasite's ability to incorporate [ 3 H]hypoxanthine. However, while DS4 was toxic toward both the parasite and the host erythrocyte, DS3 was toxic only toward the intraerythrocytic parasite. To gain insight into the mechanism of this selective cytotoxicity, we labeled the peptides with fluorescent probes and investigated their organization in solution and in membranes. In Plasmodium-infected cells, rhodamine-labeled peptides interacted directly with the intracellular parasite, in contrast to noninfected cells, where the peptides remained bound to the erythrocyte plasma membrane. Binding experiments to phospholipid membranes revealed that DS3 and DS4 had similar binding characteristics. Membrane permeation studies indicated that the peptides were equally potent in permeating phosphatidylserine/phosphatidylcholine vesicles, whereas DS4 was more permeative with phosphatidylcholine vesicles. In aqueous solutions, DS4 was found to be in a higher aggregation state. Nevertheless, both DS3 and DS4 spontaneously dissociated to monomers upon interaction with vesicles, albeit with different kinetics. In light of these results, we propose a mechanism by which dermaseptins permeate cells and affect intraerythrocytic parasites.

Single stranded DNA (s.s.DNA) comprising 1-2% of the total nuclear DNA was isolated by an improved method of hydroxyapatite chromatography from native nuclear DNA3 of embryonic chick cells, labeled for several cell generations with 3H-thymidine. Small quantities of 3H-DNA were annealed with a large excess of unlabeled DNA or polysomal RNA from chick embryos. Hybridization kinetics (monitored by the use of SI nuclease digestion, hydroxyapatite chromatography and thermalfusion), indicated that s.s.DNA belongs to the non repetitious fraction of the cell genome. One third represents DNA sequences engaged in the transcription of messenger RNA's.

By an improved method of hydroxylapatite chromatography, the reassociated sequences of chick nuclear DNA were isolated, and their base composition analysed. By increasing the amount of reassociation, the G + C content of the renatured sequences decreased progressively to reach a mean value corresponding to that of the total DNA. In order to study the distribution of the families, or group of families having different amount of reassociation, DNA was fractionated by CsC1 density gradient centrifugation. Fractions having different G + C content were obtained, and their reassociation rates analysed. At high C(o)t value of renaturation (C(o)t=50) the amount of reassociated sequences included in the high or in the low buoyant density DNA fractions was approximately the same, but their G + C content was as expected different. At lower C(o)t values of renaturation (between C(o)t of 0.2 and the C(o)t of 10), the results indicated an heterogeneity of the repeated sequences in the A + T rich DNA fractions, as compared to the G + C rich ones.

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SUMMARYA minor fraction of single-stranded DNA (ssDNA) was isolated by an improved method of hydroxylapatite chromatography (HAC) from the native nuclear DNA (nDNA) of SV-3T 3 cells, non-productively transformed by SV4o. Molecular hybridization, monitored by the use of $1 nuclease, HAC, isopycnic centrifugation and thermal melting showed that ssDNA from SV-3T3 cells (which amounts to 1.5 to 2 ~o of the total nDNA) has the same characteristics as ssDNA previously isolated from other cell species. Only 27 to 28 ~/o of ssDNA can be selfhybridized but the greatest part can be reassociated to the non-repetitive portion of nDNA and up to 38 % hybridized to homologous RNAs, as compared with 7 to 8 % for bulk nDNA. Highly radioactive virus probes (SV4o-ZH-cRNA synthesized in a cell-free system and the separated 'early' and 'late' strands of SV4o DNA labelled with a25I) were annealed to different excess amounts of cellular DNA. Both the quantities of each probe hybridized at saturation levels and the various reaction kinetics indicated that ssDNA is greatly enriched for virus sequences, mainly originating from the 'early' DNA strand which is predominantly expressed in SV-3T3 cells. The mode of formation of ssDNA is discussed in the light of other findings on the effects of DNA untwisting proteins and susceptibility of active animal genes to selective enzymic attacks.