Deletion/frameshift mutation in the α1 null allele,PI*QObolton

Summary The most common deficiency allele of the protease inhibitor (PI) ₁ (1AT) is PI*Z. Other rare deficiency alleles of 1AT are of two types: those producing low but detectable amounts of 1AT (<20% of normal serum concentrations), and null alleles producing <1% of normal 1AT and therefore not detectable by routine quantitative methods. We have previously used DNA polymorphisms and family data to determine heterozygosity in an individual producing low levels of serum 1AT (12% of normal) of PI type Mmalton. By DNA analysis we observed the typical haplotype associated with PI* Mmalton and a unique null haplotype associated with the allele PI*QObolton. The QObolton allele produces no detectable serum 1AT. We have cloned and sequenced the QObolton allele from a phage genomic library. Deletion of a single cytosine residue near the active site of 1AT in exon V results in a frameshift causing an in-frame stop codon downstream of the deletion. This stop codon leads to premature termination of protein translation at amino acid 373, resulting in a truncated protein. The truncated protein is predicted to have an altered carboxy terminus (amino acids 363-) and will lack structurally important amino acids.     

Identification of an indigenous plasmid carrying a gene for trimethoprim resistance in Pseudomonas syringae pv. glycinea

Summary Pseudomonas syringae pv. glycinea Race 8 strain PgB3 is naturally resistant to trimethoprim (Tp) at concentrations up to 500 g/ml. A genomic library of total PgB3 DNA was constructed by ligating EcoRI-restricted DNA into the EcoRI site of the cosmid vector, pLAFR1, packaging the DNA in vitro into bacteriophage lambda, and transducing E. coli DH1 cells. Of 960 cosmid clones selected for resistance to tetracycline, six were resistant to trimethoprim at 500 g/ml. An insert into pLAFR1 of about 9.4 kb was shown to be consistently present in the tirmethoprim-resistant clones. Southern blot analysis using radioactively labeled insert DNA as probe indicated that the 9.4 kb fragment hybridized only with a 40 kb indigenous plasmid from PgB3 designated pPg2.     

Phylogenetic relationships between cultivated and wild species of the genusBeta revealed by DNA “fingerprinting”

Forty-one accessions of the genusBeta representing wild and cultivated species of all sections were analyzed by DNA fingerprinting. Four sugar beet minisatellite DNA probes revealed characteristic banding patterns with Southern-hybridizedBeta DNA restricted withHindIII. A total of 111 polymorphic RFLP bands were scored across all accessions. Cluster analysis based on genetic similarity estimates for all 820 combinations of accessions revealed the following results. (1) All accessions could unambiguously be identified by a characteristic RFLP banding pattern. (2) The sugar beet cultivars examined displayed a low level of genetic diversity; they showed high similarity toB. Vulgaris ssp.maritima but low genetic similarity to the other wild species of section I. (3) In most cases, the present taxonomic classification of the genusBeta was confirmed. Species of sections II, III, and IV were clearly distinguishable from those of section I except forB. Macrocarpa, which showed high similarity to wild species of section II. In a second experiment, 108 single-copy RFLP probes from sugar beet were Southern hybridized withB. procumbens DNA. A surprisingly low degree of homology (34%) was found. The results are discussed with regard to the taxonomic classification of the genusBeta.     

Unusual behaviour of SPO1 DNA with respect to restriction and modification enzymes recognizing the sequence 5''-G-G-C-C

Summary SPO1 DNA contains only 5 cleavage sites for restriction enzymes which recognize and cleave the sequence 5-G-G-C-C (HaeIII or BsuR). Fragments of SPO1 DNA cloned in E. coli to substitute 5-hydroxymethyluracil (HMU) by thymine (T) remain resistant to HaeIII indicating that this unexpectedly small number of cleavages by HaeIII is not correlated with the presence of HMU in the normal phage DNA. It was previously shown that SPO1 is neither subject to B. subtilis R restriction (Trautner et al., 1974) nor modification in vivo (Günthert et al., 1975). We now show that SPO1 DNA can however be restricted and modified in vitro.     

An electrochemical analysis of changes in properties of DNA caused by ionizing and ultraviolet radiations

Summary Electrooxidation and electroreduction of- and u.v.-irradiated DNA were studied by means of differential pulse voltammetry at the graphite electrode and differential pulse polarography at the dropping mercury electrode. Two separated voltammetric oxidation peaks G and A were used for monitoring conformational changes in guanine - cytosine (GC) and adenine - thymine (AT) pairs respectively in irradiated double-stranded (ds) DNA. Pulse-polarography reduction peak III was used for detection of denatured DNA in the irradiated samples of ds DNA. It was found that the heights of peaks G and A of ds DNA did not change with the radiation dose after relatively low doses of- and u.v.-radiations (up to ca. 40 krads and 1 × 10⁴ Jm^–2, respectively), when no single-stranded (ss) DNA was detected in the irradiated DNA samples. After higher doses of radiation the occurrence of ss DNA or ss segments in the irradiated samples of ds DNA was accompanied by an increase of peaks G and A; however, peak A grew more rapidly with the increasing dose than peak G. It was concluded that the results obtained support the assumption, according to which regions of ds DNA rich in AT pairs are more susceptible to denaturation caused by- and u.v.-radiations.This dose concerns the DNA solution at a concentration of 600 µg/ml^–1     

Photodynamic effect of proflavine on ?X174 bacteriophage, its DNA replicative form and its isolated single-stranded DNA: Inactivation, mutagenesis and repair

Summary In contrast to that what is observed with most inactivating agents, proflavine-mediated photoinactivation is about 10 times more efficient on double-stranded X 174 replicative form DNA (RFI) than on isolated single-stranded X 174 DNA. Both X RFI DNA and encapsidated DNA have similar sensitivities to proflavine and light treatment.With the three substrates studied, reactivation can occur through high multiplicity of infection and depends upon the cellular rec A gene product. No effect of the pol A, uvr A or lex A gene mutations has been found on either phage or DNA inactivation rates.The photodynamically induced lesions can be repaired at least in part, by the SOS repair system induced in the host-cells by a 100 J ·m^-2 UV irradiation. SOS repair does not occur with bacteria (or spheroplasts) irradiated in the presence of chloramphenicol.Reversion frequency of the X 174 amber mutations indicates that 1) photodynamically induced lesions are mutagenic whether the rec A gene product is present or not in the indicator bacteria; 2) induction of the SOS repair system is accompanied by a mutagenic process which almost results in a two fold increase of the reversion frequency; and 3) multiplicity reactivation occurs through a recombinational process and is not mutagenic per se.     

Virulence as a consequence of genome instability of a novel temperate bacteriophage, ΦRsG1, of Rhodobacter sphaeroides Y

A novel temperate bacteriophage, designated RsG1, was isolated from Rhodobacter sphaeroides Y (previously designated Rhodopseudomonas sphaeroides) following exposure to mitomycin C. The phage morphology, as revealed from electron microscopy, showed a hexagonal head (90 by 46.5 nm) connected with a tail (116 by 9.4 nm), to which a collar was proximally attached. A morphologically similar phage was also produced by spontaneous lysis of the cells. While RsG1 did not grow on any other bacterial strain tested, spontaneously produced phage particles propagated (and formed plaques) on R. sphaeroides Y still carrying RsG1 in the prophage state. The genome of RsG1 consisted of double stranded linear DNA with cohesive ends and a GC-content of 71.8 mol%. The DNA molecules formed circles in vitro with a mean contour length of 17.18±0.4 m, which corresponds to a size of 49 kbase pairs (kb). On the other hand, DNA extracted from the virulent phage particles was heterogeneous and consisted of two DNA species of different size, occurring in a ratio of about 1:1. These molecules also circularized having contour lengths of 17.18±0.4 m and 14.02±0.41 m corresponding to 49 and 40 kb, respectively. Restriction digest analysis of the two DNA species and DNA from RsG1 indicated that they are similar, and allowed the indentification of an 11.5 kb EcoRI fragment that carries the cohesive ends. Because DNA from RsG1 and the 49 kb DNA of the virulent phage particles were indistinguishable with the criteria applied, it is suggested that phage particles containing the 40 kb DNA represent the virulent type of phage, termed RsG1.1.     

On the dynamics of DNA in aqueous solution. Pulse radiolysis studies using the light scattering detection method

Summary The kinetics of DNA chain breakage in solution induced by 2 µs pulses of 15 MeV electrons were investigated by light scattering. On irradiating native calf thymus DNA at room temperature the decrease of light scattering intensity (LSI) - due to double strand ruptures - shows a fast decay with a half life _1/2 of about 30 ms as well as a slow decay with _1/2 of about 10 s. With increasing temperature (20–40° C) both the total degree of degradation and the fraction of the fast decay increase due to the facilitated melting of segments between two single strand breaks on alternate strands forming a double strand break. Above 40° C a third mode of LSI decay with _1/2 of 5–10 s arises, indicating detachment of relatively long segments.The total relative decrease of LSI after irradiation A, which can be taken as a measure of the degree of degradation, follows the square of the absorbed dose in the case of native DNA, whereas on irradiating denatured DNA A rises linearly with dose. The decay of LSI due to the degradation of denatured DNA is much faster than that of native DNA with _1/2 down to 150 µs, depending on the absorbed dose. The half lives are interpreted in terms of the separation of fragments by diffusion and of the melting of double strand segments between two single strand breaks.     

Prophage induction and cell division in E. coli. II. Linked (recA, zab) and unlinked (lex) suppressors of tif-1-mediated induction and filamentation

Summary The pleiotropy of tif-1, a mutation in E. coli K12 causing, among other effects, cellular filamentation at 42° and thermal induction of lysogenic derivatives, can be explained by the participation of the tif ⁺ gene product in more than one reaction pathway. Pathways that involve the tif ⁺ product may be analyzed by selection of secondary mutations that reverse both tif-1-mediated prophage induction and cell filamentation. Among revertants of a tif-1 lysogen among 20% are recombination deficient. These appear to carry a recA mutation. In addition to this class is a rarer (7%) phenotypically distinguishable class of revertants, called zab, first described here. Markers tif-1, recA and zab are closely linked. Mutations lex which are dominant and located near malB also appear (3%) among tif-1 revertants. The lex ⁺ function is needed for normal UV, X-ray and mitomycin C induction of prophage .The zab mutation resembles recA in causing (1) high sensitivity to UV, X-rays and mitomycin C, (2) drastic DNA degradation following UV irradiation but normal capacity to repair UV-damaged infecting phage (Hcr⁺), (3) failure to carry out UV reactivation and UV mutagenesis of UV-irradiated bacteriophage , (4) a markedly reduced level of spontaneous induction of . In contrast, other capacities, strikingly diminished by recA, are affected less, if at all by zab. Thus zab (1) permits 30–60% normal recombination proficiency, (2) shows real, although very low inducibility of by UV or mitomycin C, (3) permits 100% efficiency of plating of red gam, and (4) does not degrade DNA spontaneously.The hypothesis is proposed that the tif-1 mutation is a regulatory mutation controlling the activity, or more likely the synthesis of repair enzyme(s). The level of these repair enzyme(s), rather than DNA lesions, may govern the stability of the prophage repressor and the capacity of the bacteria to form septa.     

Probing the interactions of the solvated electron with DNA by molecular dynamics simulations: II. bromodeoxyuridine-thymidine mismatched DNA

The interaction of solvated electrons with DNA results in various types of DNA lesions. The in vitro and in vivo sensitisation of DNA to -induced damage is achieved by incorporation of the electron-affinity radiosensitiser bromodeoxyuridine (BUdR) in place of thymidine. However, in DNA duplexes containing single-stranded regions (bulged BUdR-DNA), the type of lesion is different and the efficiency of damage is enhanced. In particular, DNA interstrand crosslinks (ICL) form at high efficiency in bulged DNA but are not detectable in completely duplex DNA. Knowledge about the processes and interactions leading to these differences is obscure. Previously, we addressed the problem by applying molecular modelling and molecular dynamics (MD) simulations to a system of normal (BUdR·A)-DNA and a hydrated electron, where the excess electron was modelled as a localised eˉ(H₂O)₆ anionic cluster. The goal of the present study was to apply the same MD simulation to a wobble system, containing a pyrimidine–pyrimidine mismatched base pair, BUdR·T. The results show an overall dynamic pattern similar to that of the motion around normal DNA. However, the number of configuration states when was particularly close to DNA is different. Moreover, in the (BUdR·T)-wobble DNA system, the electron frequently approaches the brominated strand, including BUdR, which was not observed with the normal (BUdR·A)-DNA. The structure and exchange of water at the sites of immobilisation near DNA were also characterised. The structural dynamics of the wobble DNA is prone to more extensive perturbations, including frequent formation of cross-strand (cs) interatomic contacts. The structural deviations correlated with approaching DNA from the major groove side, with sodium ions trapped deep in the minor groove. Altogether, the obtained results confirm and/or throw light on dynamic-structure determinants possibly responsible for the enhanced radiation damage of wobble DNA. Figure The structure of the tightly bound single water-layer between the DNA and the electron (Site-8, five H₂O molecules, bold capped sticks); the rest of the “second” shell waters (lines, in atom type colour) surround the ˉ(H₂O)₆ cluster (yellow, space fill). Orange dashed lines H-bonds; only one of the five molecules from the single H₂O layer mediates a single-step H-bond bridge with N7(A8); the other four present a network of two(three)-step H-bond bridges between DNA/ partner atoms     

Regional replication of the bacterial chromosome induced by derepression of prophage lambda

Summary We have demonstrated previously by DNA-DNA hybridization that induction of phage with wild type O and P genes results in an increase of bacterial DNA in the chromosomal region adjacent to the left of the prophage, that is a segment between gal and att (gal DNA) (Imae and Fukasawa, 1970). Evidence is presented in this report that such an increase of bacterial DNA is also seen in the region to the right of the prophage; a segment between bio and att (bio DNA). We postulate therefore that the bidirectional replication of DNA extends beyond the prophage and copies the neighboring host DNA until the prophage is excised. The model is verified by making use of excision-defective phages. The synthesis of gal DNA (or bio DNA) slows down to a halt within 40 min after the induction in the normal lysogens. The results are attributed to the prophage excision: (1) In lysogens for int, synthesis of the bacterial DNA continues for longer times. (2) The synthesis of the bacterial DNA slows down to a halt in lysogens for xis or b2 as in the control. However DNA synthesis also slows down in parallel so that the amount of the bacterial DNA relative to that of DNA synthesized by a given time stays constant from 20 min to 80 min. During that time the relative amount of the bacterial DNA rapidly decreases in the normal lysogen.The first article of this series is in J. molec. Biol. 54, 585 (1970).     

Conserved and variable regions in the chromosomal and extrachromosomal DNA of halobacteria

Summary Total DNA from Halobacterium halobium and other halobacteria strains is separated into two fractions, FI and FII, which differ in their G+C content. FI DNA, which represents the major part of the genome is highly conserved in all purple-membrane-forming halobacteria. Fraction II (FII) consists in H. halobium of three DNA specimen: (a) the previously isolated plasmid pHH1, (b) a heterogeneous set of ccc-DNA molecules present in the cell in low copies, termed minor-circular DNA (MCD) and (c) a new type of more A-T rich DNA segments (chromosomal islands) which, as described here and by Pfeifer and Betlach (1985), are integrated in FI. Sequences homologous to pHH1 occur only in Halobacterium species closely related to H. halobium (like H. cutirubrum), whereas MCD sequences are present in all purple-membrane-forming halobacteria. The sequences of the newly identified chromosomal islands are only found like pHH1, in Halobacterium species, closely related to H. halobium. Total DNA from square halobacteria exhibits no extended homologies to FI or FII DNA from H. halobium. The only common DNA sequences found in all halobacteria are certain insertion elements (ISH), such as ISH26. Based on these data, halobacteria can be subdivided in at least three major groups.Dedicated to Prof. Dr. F. Lingens to his 60th anniversary     

Human cellular sequences detectable with adenovirus probes

Previous studies suggesting homology between human cellular DNA and the DNAs from adenovirus types 2 and 5 are extended in the present paper. A clone (ChAd^h), isolated from a human genomic DNA library using an adenovirus probe, hybridized to discrete regions of adenovirus 2 DNA, including part of the transforming genes E1a and E1b, as well as to repeated sequences within human DNA. The E1a and E1b genes both hybridize to the same 300 base pair Sau3AI fragment within ChAd^h although there is no obvious homology between E1a and E1b. The Ad 2 E1a gene was also used as a probe to screen other cellular DNAs to determine whether repeated sequences detectable with Ad 2 DNA probes were conserved over long evolutionary periods. Hybridization was detected to the genomes of man, rat, mouse and fruit fly, but not to those of yeast and bacteria. In addition to a smear hybridization, discrete fragments were detected in both rodent and fruit fly DNAs. The experiments reported suggest the existence of two different types of cellular sequences detected by Ad 2 DNA: (1) repeated sequences conserved in a variety of eukaryote genomes and (2) a possible unique sequence detected with an E1a probe different from that responsible for hybridization to repeated sequences. This unique sequence was detected as an EcoRI fragment in mouse DNA and had a molecular size of about 8.8 kb.     

Transfer of Ogu cytoplasmic male sterility to Brassica juncea and improvement of the male sterile line through somatic cell fusion

Male sterility conferred by ogu cytoplasm of Raphanus sativus has been transferred to Brassica juncea cv RLM 198 from male-sterile B. napus through repeated backcrossing and selection. The male-sterile B. juncea is, however, highly chlorotic and late. It has low female (seed) fertility and small contorted pods. To rectify these defects, protoplasts of the male sterile were fused with normal RLM 198 (green, self fertile). Four dark green, completely male-sterile plants were obtained and identified as putative cybrids. All the plants were backcrossed three times with RLM 198. Mitochondrial and chloroplast DNA analysis of backcross progeny confirmed hybridity of the cytoplasm. The restriction pattern of the chloroplast DNA of progeny plants of three cybrids (Og 1, Og 2, Og 3) was similar to that of the green self-fertile RLM 198 and indicated that the correction of chlorosis resulted from chloroplast substitution. The chloroplast DNA of the lone progeny plant of the fourth cybrid (Og 10) could not be analyzed because the plant was stunted and had only a few leaves. When total cellular DNA was probed with mitochondrial probes coxI and atpA it was found that the cybrids had recombinant mitochondria. The chlorosis-corrected plants were early flowering and had vastly improved seed fertility.     

The chromosomes and DNA of Allium cepa

Giemsa C-banded idiograms that allow the identification of all chromosomes have been prepared for Allium cepa, Ornithogalum virens, and Secale cereale. An analysis of A. cepa DNA has determined that: (1) It has the lowest GC content so far reported for an angiosperm (32%). (2) It appears to have no satellite DNA detectable by CsCl or Cs₂SO₄-Ag⁺ density gradient centrifugation. (3) Aside from fold back DNA and unreactable fragments, a C₀t curve indicates that most of the DNA can be adequately described as two major middle repetitive components (Fractions I and II) and a single copy component (Fraction III). And (4) most of the repeated DNA sequences are involved in a short period interspersion pattern with single copy and other repetitive sequences. In situ hybridization of tritiated cRNAs to fold back, long repeated, and Fraction I DNA from A. cepa to squash preparations of chromosomes and nuclei from A. cepa, O. virens, and S. cereale root tips indicates: (1) Sequences complementary to fold back DNA are scattered throughout the genome of A. cepa except for telomeric heterochromatin and nucleolus organizers while they are not detectable in the genomes of O. virens or S. cereale. (2) Although long repeated sequences are scattered throughout the genome of A. cepa, they are concentrated to some extent in telomeric heterochromatin and nucleolus organizers (NOs). Sequences complementary to long repeats of A. cepa occur primarily in chromosome three of O. virens while these sequences are more common in the genome of more distantly related S. cereale. (3) Fraction I DNA is scattered throughout the genome of A. cepa while it is hardly detectable in the genomes of O. virens and S. cereale. These results are discussed in regard to the evolutionary conservation and function of repeated DNA sequences.     

Chloroplast DNA from three archegoniates

1. DNA from female and male Sphaerocarpos donnellii (liverwort) plants exhibits at least two species with buoyant densities of 1.703 (main band) and 1.691 (satellite) g cm^-3 in CsCl equilibrium gradients. At least part, if not all, of the satellite DNA is localized in plastids. It consists of up to 90% of uniformly sized circular molecules of an average circumference of 38.5 m. Compared to other Chlorophyta, the liverwort's cpDNA is unusually low both in diensity and contour length. — 2. On the hand, cpDNA from the ferns Asplenium nidus and Pteris vittata resembles those of higher plants in buoyant density (1.697 g cm^-3) and circumference (about 44.8 m). — 3. Analysis of DNA from the archegoniate chloroplasts with restriction endonucleases indicates chat the cyclic molecules are monomers. — 4. The results show that the circular molecules found in cpDNA of higher plants do not represent the functionally required minimum size of DNA in plastids.Abbreviations cpDNA chloroplast - DNA nucDNA=nuclear - DNA Sal I=restriction endonuclease from Streptomyces albus S - Eco RI restriction endonuclease from Escherichia coli, carrying resistance factor 1 - DTT dithiothreitol (Cleland's reagent) - Saline-EDTA 0.15 M NaCl, 0.1 M ethylene diamine tetraacetic acid, pH 8.0 - SSC 0.15 M NaCl, 0.015 M Na citrate, pH 7.2 - DNAase deoxyribonuclease - Md Megadalton Dedicated to the memory of Prof. Dr. Edgar Knapp     

Chromosomal unit fibers in Drosophila

Chromosomal unit fibers consisting of long, regular fibers of about 0.40 m diameter were obtained from disintegrated, isolated chromosomes of two Drosophila melanogaster cell lines. In one cell line with an essentially normal karyotype, three clearly defined size classes of 15, 13, and 11 m length were observed corresponding to the three larger chromosomes of Drosophila. In a cell line carrying an additional translocation between the two largest chromosomes a 19 m fiber derived from the translocation chromosome was observed. Direct determinations of the DNA content per m length of Drosophila unit fibers show that DNA is contracted by a factor of about 1400x in agreement with calculations based on the length of the unit fibers and the known DNA content of the individual Drosophila chromosomes. These findings support our previously proposed model for the unit fiber sub-structure of chromosomes as being derived by a hierarchy of coiling with the corresponding contraction ratios being 7 (100 Å string of nucleosomes), 5 to 6 (250–300 Å thick nucleohistone fiber), and about 40 (unit fiber), resulting in a total contraction of DNA in unit fibers in the order of 1400x.     

Replicon fusion mediated by a single-ended derivative of transposon Tn1721

Summary Tn17221K, a derivative of transposon Tn1721 lacking one terminal inverted repeat (IR) and conferring kanamycin resistance, promotes transposition of the resistance marker to a target replicon at about 100-fold lower frequency than the wild-type element. A study involving restriction analysis of 16 independent Tn17221K-mediated events led to the following results: (i) Tn17221K mediates fusions of the donor (pRU506) and target (RSF1010) replicons; the fused entities are non-permuted. (ii) Tn17221K promotes insertions of donor DNA at many different sites in the target replicon. (iii) The analyzed fusion plasmids contain the entire target and various lengths of donor DNA. Eleven products contain the entire donor plasmid plus a duplication of the IR (class A), whereas five products contain only portions adjacent to the single IR (class B). (iv) In each case the two replicons are joined at (or very close to) the single IR. The second junction is located shortly beyond the duplicated IR in class A and at different sites within the donor plasmid in class B. These results are interpreted in terms of asymmetric replicative transposition.     

Nuclear differentiation and nucleic acid synthesis in well-fed exconjugants of Paramecium aurelia

Paramecium aurelia exconjugants contain new macronuclear anlagen and numerous fragments of the old pre-zygotic macronucleus. Macronuclear anlagen develop during the first two cell cycles after conjugation. During this time their volume increases from about 11 m³ to about 3700 m³ and more than 10 doublings of DNA content occur. The rate of DNA synthesis is between two and three times as great as in the vegetative macronucleus. — In macronuclear fragments, however, DNA synthesis is suppressed. The rate of DNA synthesis in macronuclear fragments during the extended first cell cycle after conjugation (11 1/2 hr. vs. 5 1/2 hr. for the vegetative cell cycle) is only about one-third of the rate in vegetative macronuclei and there is only a 65% increase in the mean DNA content of fragments. The rate of fragment DNA synthesis continues to decrease during each of the subsequent two cell cycles. — Unlike the rate of DNA synthesis, the rate of RNA synthesis per unit of DNA is similar in macronuclear anlagen, macronuclear fragments and fully developed macronuclei. Macronuclear fragments continue to synthesize RNA at the normal rate long after the new macronuclei are fully developed. Fragments contribute about 80% of all RNA synthesized during the first two cell cycles after conjugation. RNA synthesis begins very early in the development of macronuclear anlagen and nucleolar material appears during the first half-hour of anlage development. — Chromosome-like structures were never observed during anlage development and there was no evidence of two periods of DNA synthesis separated by a DNA poor stage as has been observed in several hypotrichous Ciliates.