Cell cycle-dependent induction of mutations along a yeast chromosome

Simultaneous induction of multiple mutations by N-methyl-N'-nitro-N-nitrosoguanidine in the yeast Saccharomyces cerevisiae

Contrary to what happens in bacteria, mutations induced by nitrosoguanidine in yeast are not accompanied by an excess of mutations in nearby genes. We have investigated nitrosoguanidine mutagenesis in three regions of the yeast genome: the contiguous DNA segments HIS4A, HIS4B and HIS4C, located on chromosome III; ADE1 and CDC15 separated by about 3 map units on chromosome I; and CAN1, some 50 map units away from the centromere on chromosome V. Revertants at HIS4C never suffered mutations at HIS4A or HIS4B. Reversion at CDC15 did not affect the frequency of mutation at ADE1. No tsm mutations, leading to thermonsensitivity, were found in the immediate vicinity of the locus CAN1 after selecting for canavanine resistant mutants. However, as expected from nitrosoguanidine mutagenesis of replication points and the fixed pattern of chromosome replication, the induced tsm mutations seem not to map randomly over the yeast genome; in fact, two out of the three groups of such tsm mutations studied are located in the same chromosome arm as CAN1, indicating that these two regions are replicated at the same time as CAN1. Replication synchrony is less than perfect, since the tsm mutations of each group affect many different genes.

Replication timing: histone genes replicate during early S phase in cleavage-stage embryos of sea urchin

Newly synthesized DNA was separated from the bulk of the DNA by pulse-labeling with BUdR and centrifugation in an alkaline CsCl buoyant density gradient. The content of histone gene in the newly synthesized DNA was determined by DNA dot hybridization. The gene contents in DNA replicated during the early half of S phase and during the whole S phase were compared. Results showed that histone genes were replicated during the first half of the S phase in embryos in the early cleavage stage.

Temporal order of replication of genes responsible for hypoxanthine phosphoribosyl transferase and Na+/K+ ATPase in chemically transformed human fibroblasts

The cytotoxic and mutagenic effects of a direct perturbation of DNA during various portions of the DNA synthetic period (S phase) of a chemically induced, transformed line (Hut-11A cells) derived from diploid human skin fibroblasts were examined. The cells were synchronized by a period of growth in low serum with a subsequent blockage of the cells at the G1/S boundary by hydroxyurea. This method resulted in over 90% synchrony, although approximately 20% of the cells were noncycling. Synchronized cells were treated for each of four 2-h periods during the S phase with 5-bromodeoxyuridine (BrdU) followed by irradiation with near-ultraviolet (UV). The BrdU-plus-irradiation treatment was cytotoxic and mutagenic, while treatment with BrdU alone or irradiation alone was neither cytotoxic nor mutagenic. The cytotoxicity was dependent upon the periods of S phase during which treatment was administered. The highest lethality was observed for treatment in early to middle S phase, particularly in the first 2 h of S phase, whereas scare lethality was observed in late S phase. The BrdU-plus-irradiation treatment induced ouabain- and 6-thioguanine-resistant mutants, while BrdU alone or irradiation alone was not mutagenic. Ouabain-resistant mutants were induced during early S phase by the BrdU-plus-irradiation treatment. 6-Thioguanine-resistant mutants, however, were induced during middle to late S phase. These results suggest that a certain region or regions in the DNA of Hut-11A cells, as designated by their specific temporal relationship in the S phase, may be more sensitive to the DNA perturbation by BrdU treatment plus near-UV irradiation for cell survival and that gene(s) responsible for Na+/K+ ATPase is replicated during early S phase and gene(s) for hypoxanthine phosphoribosyl transferase is replicated during middle to late S phase.

Regulation of the RAD6 gene of Saccharomyces cerevisiae in the mitotic cell cycle and in meiosis

The regulation of the RAD6 gene at the mRNA level was investigated. The level of steady state RAD6 mRNA increases once every cell cycle, at late S/early G2. This stage is the one at which rad6 mutants arrest, as do wild-type cells exposed to hydroxyurea (HU) or methyl methanesulfonate (MMS), or cdc40 cells exposed to the restrictive temperature. This appears to be a repair-specific stage in the cell cycle. RAD6 mRNA levels increase when cells are treated with MMS, but this increase seems to be due to the arrest of the cells by MMS at the repair-specific stage; cells arrested at the same stage by HU or by the cdc40 lesion also show high levels of RAD6 mRNA. A much smaller increase in the level of RAD6 transcripts is seen following UV irradiation. During meiosis, RAD6 mRNA is more abundant before commitment to recombination. The differential increase of RAD6 mRNA during the S/G2 repair-specific stage of the cell cycle relates the RAD6 function to the normally occurring radioresistance found at this stage.

Alkylation mutagenesis in Saccharomyces cerevisiae: lack of evidence for an adaptive response

We have found no evidence for an adaptive response for either lethality or mutagenesis following treatment of Saccharomyces cerevisiae with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The rad6 and rad52 mutants of S. cerevisiae are highly defective in MNNG and ethyl methanesulfonate induced mutagenesis of both stationary and exponential phase cells. These and other observations indicate that the mechanisms of repair of alkylation damage and mutagenesis differ markedly between S. cerevisiae and Escherichia coli.

Control of cell growth and division in Saccharomyces cerevisiae

Considerable advances have been made in recent years in our understanding of the biochemistry of protein and nucleic acid synthesis and, particularly, the molecular biology of gene expression in eukaryotes. The yeast Saccharomyces cerevisiae, and to a lesser extent Schizosaccharomyces pombe, has had a preeminent role as a focus for these studies, principally because of the facility with which these organisms can be experimentally manipulated biochemically and genetically. This review will be designed to critically examine and integrate recent advances in several vital areas of regulatory control of enzyme synthesis in yeast: structure and organization of DNA, transcriptional regulation, post-transcriptional modification, control of translation, post-translational modification and secretion, and cell-cycle modulation. It will attempt to emphasize and illustrate, where detailed information is available, principal underlying molecular mechanisms, and it will attempt to make relevant comparisons of this material to inferred and demonstrated facets of regulatory control of enzyme and protein synthesis in higher eukaryotes.

Dependence of lethality induced by a direct DNA perturbation of synchronized human diploid fibroblasts on different periods of the DNA synthetic period (S phase)

The cytotoxic effect of a direct perturbation of DNA during various portions of the DNA synthetic period (S phase) of cultured human diploid fibroblasts was examined. The cells were synchronized by a period of growth in low serum with a subsequent blockage of the cells at the G1/S boundary by hydroxyurea. This method resulted in over 90% synchrony, although approximately 20% of the cells were noncycling. Synchronized cells were treated for each of four 2-hour periods during the S phase with 5-bromodeoxyuridine (0.1-10 microM), followed by irradiation with near-UV (5-10 min). The 5-bromodeoxyuridine-plus-irradiation treatment was cytotoxic, while treatment with 5-bromodeoxyuridine alone or irradiation alone was not cytotoxic. The cytotoxicity was dependent upon the periods of S phase during which treatment was administered. The highest lethality was observed for treatment in early to middle S phase, particularly in the first 2 hours of S phase, whereas scarce lethality was observed in late S phase. The extent of substitution of 5-bromodeoxyuridine for thymidine in newly synthesized DNA was similar in every period of the S phase. Furthermore, no specific period during S phase was significantly more sensitive to treatment with respect to DNA damage, as determined by an induction of unscheduled DNA synthesis. These results suggest that a certain region or regions in the DNA of human diploid fibroblasts, as designated by their specific temporal relationship in the S phase, may be more sensitive to the DNA perturbation by 5-bromodeoxyuridine treatment plus near-UV irradiation for cell survival.

A family of Saccharomyces cerevisiae repetitive autonomously replicating sequences that have very similar genomic environments

We have characterized a family of moderately repetitive autonomously replicating sequences (ARSs) in Saccharomyces cerevisiae. Restriction mapping, deletion studies and hybridization studies suggest that these ARSs, which are probably less than 350 base-pairs in size, share one common feature: each is located close to, but not within, a repetitive sequence (131) of approximately 10(3) to approximately 1.5 X 10(3) base-pairs in length. These ARSs can be divided into two classes (X and Y) by their sequence homology and genomic environments. Each of the class X ARSs is embedded within a repetitive sequence (X) of variable length (approximately 0.3 X 10(3) to approximately 3.75 X 10(3) base-pairs); each of the class Y ARSs is embedded within a highly conserved repetitive sequence (Y) of approximately 5.2 X 10(3) base-pairs in length. Both of these sequences are located directly adjacent to the 131 sequence.

Differential cell cycle phase specificity for neoplastic transformation and mutation to ouabain resistance induced by N-methyl-N'-nitro-N-nitrosoguanidine in synchronized C3H10T 1/2 C18 cells

The transformable mouse embryo fibroblast cell line C3H10T 1/2 C18 has been employed to study the induction by the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) of morphological transformation and mutation to ouabain resistance throughout the cell cycle. Cells were synchronized by means of isoleucine deprivation for 24 hr and initiated DNA synthesis with a high degree of synchrony 7.5 hr after release of the isoleucine block. At various intervals throughout the cell cycle cultures were treated with MNNG at 1.0 microgram/ml and the induction of cytotoxicity, morphological transformation, and ouabain-resistant colonies was determined. All three phenomena exhibited marked cell-cycle phase dependency. Maximal induction of transformation occurred in cultured treated 7.5 hr after release from isoleucine deprivation, when the cells were at the G1/S boundary. In contrast, induction of ouabain-resistant colonies was at a minimum at the time of maximal induction of transformation, and peak induction of ouabain resistance did not occur until 16-18 hr after release from the isoleucine block, when cells were in late S phase. A close correlation was observed between the induction of cytotoxicity and of ouabain-resistant mutants. The results suggest that differences exist in the production or cellular processing of the various early lesions.

Origin of replication from Xenopus laevis mitochondrial DNA promotes high-frequency transformation of yeast

A specific fraction of chromosomal DNA from both yeast and a wide variety of other eukaryotes, but not from Escherichia coli, promotes high-frequency transformation in yeast. The plasmids containing these sequences are maintained as extra-chromosomal molecules in transformed cells. These results suggest that similar or identical sequences are used for the initiation of DNA replication in eukaryotes. To test this hypothesis, several foreign eukaryotic DNAs implicated directly or indirectly in the initiation of DNA replication have been examined for their ability to promote autonomous, extrachromosomal replication in yeast. Simian virus 40 DNA, amplified Xenopus laevis ribosomal DNA, X. laevis 5S ribosomal DNA, X. laevis mtDNA, and five different members of the Alu I family of human middle repetitive DNAs were cloned into the vector YIp5 and used to transform yeast. Of these DNAs, only Xenopus mtDNA promoted high-frequency transformation and extrachromosomal maintenance of YIp5 DNA. A 2.2-kilobase EcoRI fragment from the 17.4-kilobase mtDNA molecule was responsible for these activities. This fragment contains the sequence used for the initiation of replication in Xenopus mitochondria.

Autonomously replicating sequences in Saccharomyces cerevisiae

A method is presented for isolating DNA segments capable of autonomous replication from Saccharomyces cerevisiae chromosomal DNA based on the differential transforming ability of autonomously replicating plasmids and nonreplicating plasmids. DNA plasmids that are capable of self-replication in yeast transform yeast spheroplasts at about 1000-fold higher frequency than nonreplicating plasmids. We have cloned from total yeast DNA a number of DNA segments that permit the pBR322 plasmid carrying the yeast LEU2 gene to replicate autonomously. These plasmid clones are characterized by their ability to transform Leu- spheroplasts to Leu+ at a high frequency and their ability to replicate autonomously. Analysis of the insert DNAs carried in some of these self-replicating plasmids divides them into two categories: those that are unique in the yeast genome and those that are repetitive.

Regulatory mutations affecting ornithine decarboxylase activity in Saccharomyces cerevisiae

We isolated several strains of Saccharomyces cerevisiae containing mutations mapping at a single chromosomal gene (spe10); these strains are defective in the decarboxylation of L-ornithine to form putrescine and consequently do not synthesize spermidine and spermine. The growth of one of these mutants was completely eliminated in a polyamine-deficient medium; the growth rate was restored to normal if putrescine, spermidine, or spermine was added. spe10 is not linked to spe2 (adenosylmethionine decarboxylase) or spe3 (putrescine aminopropyltransferase [spermidine synthease]). spe 10 is probably a regulatory gene rather than the structural gene for ornithine decarboxylase, since we isolated two different mutations which bypassed spe10 mutants; these were spe4, an unliked recessive mutation, and spe40, a dominant mutation linked to spe10. Both spe4 and spe40 mutants exhibited a deficiency of spermidine aminopropyltransferase (spermine synthase), but not of putrescine aminopropyltransferase. This suggests that ornithine decarboxylase activity is negatively controlled by the presence of spermidine aminopropyltransferase.

Characterization of thermosensitive mutants of Physarum polycephalum a plasmodial screening method for cell cycle mutants based upon synchronization at the restrictive temperature

A screening procedure for cell cycle mutants among is mutants of Physarum has been developed for unsynchronized microplasmodia. The synchronization of the microplasmodia and the ratio of pre- to post-mitotic nuclei were evaluated after a transient shift-up to the non-permissive temperature. In contrast with wild type and is mutants not blocked in a cell cycle event, putative cell cycle mutants were synchronized. Among them, three strains which showed a reduction of DNA synthesis had an abnormally increased ratio of pre- to post-mitotic nuclei. Thus, this methodology seems to be efficient and more rapid than the previously published procedures for detecting cell-cycle mutants of Physarum.

Replication fork rate and origin activation during the S phase of Saccharomyces cerevisiae

When the growth rate of the yeast Saccharomyces cerevisiae is limited with various nitrogen sources, the duration of the S phase is proportional to cell cycle length over a fourfold range of growth rates (C.J. Rivin and W. L. Fangman, 1980, J. Cell Biol. 85:96-107). Molecular parameters of the S phases of these cells were examined by DNA fiber autoradiography. Changes in replication fork rate account completely for the changes in S-phase duration. No changes in origin-to-origin distances were detected. In addition, it was found that while most adjacent replication origins are activated within a few minutes of each other, new activations occur throughout the S phase.

Isolation and characterization of Saccharomyces cerevisiae mutants deficient in S-adenosylmethionine decarboxylase, spermidine, and spermine

Four mutants were isolated from Saccharomyces cerevisiae that are deficient in S-adenosylmethionine decarboxylase (spe2). All four mutants are chromosomal and fall into a single complementation group tightly linked to arg1. Since one of the mutants contained a temperature-sensitive activity, this complementation group defines the structural gene. Mutants totally lacking enzymic activity did not contain spermidine or spermine and had a greatly increased doubling time when grown in the absence of these two polyamines. Addition of 10(-6) M spermidine or 10(-5) M spermine, but not putrescine or cadaverine, restored the doubling time to that of the wild type. Diploids formed from a cross of two mutants completely deficient in spermidine and spermine were unable to sporulate in the absence of added spermidine or spermine. We obtained evidence that arg1 was not located on any of the 17 known chromosomes, and therefore we postulate that arg1 and spe2 are located on a new 18th chromosome.

[Rhodosporidium Banno: dose-effect relations, mutagenic efficiency, and spectrum of mutants in the induction of auxotrophic mutants by ultraviolet light and N-methyl-N'-nitro-N-nitrosoguanidine]

The kinetics, efficiency, and specificity of induction of forward mutations to auxotrophy by ultraviolet light (UV) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was examined in stationary phase cells of Rhodosporidium (Rhodotorula) wild strain Rg1. In comparison to the spontaneous level the frequency of auxotrophic mutants was increased more than 1000 times by both mutagens, however, the mutagenic efficiency of MNNG was higher than that of UV. We found that the forward mutation rate is a linear function of the applicated UV and MNNG doses in the range to 600 J m-2 or 25 mM X min, respectively. The 35 studied biosynthetic pathways to amino acids, purines, pyrimidines, and vitamins are genetically blocked at different frequencies, but there is not any significant difference between UV and MNNG induced frequencies of mutants with a specific requirement. However, in difference to the approximately equal distribution of the MNNG-induced nic mutants among the genetic blocks of the tryptophan-nicotinamide pathway, UV-induced nic mutants occurred with a higher frequency in the genes of the tryptophan pyrrolase and the 3-hydroxykynureninase than in the genes of the other enzymes of the pathway.

Identifying sites of simultaneous DNA replication in eukaryotes by N-methyl-N'-nitro-N-nitrosoguanidine multiple mutagenesis

N-methyl-N'-nitro-N-nitrosoguanidine (NG) induces certain classes of multiple mutations in yeast at high frequency. By selecting for mutation at one locus (his4 or leu1) one frequently obtains double mutants where another mutation to temperature sensitivity has also been induced. This multiple mutagenesis exhibits a considerable specificity: for mutation at one particular locus there is a high chance that another mutation will be found in the same cell at one of a restricted number of other loci. For any given locus (e.g. his4) there is a spectrum of sites at which temperature-sensitivity mutations are co-induced. This spectrum differs for different loci, such that the spectrum of sites co-mutating with leu1 differs completely from that for sites co-mutating with his4. This NG'induced co-mutation is interpreted in terms of NG acting to enhance mutagenesis at sites of simultaneous DNA replication within the cell. The results so obtained indicate a very strict control over the order and timing of gene replication in Saccharomyces cerevisiae, and it is suggested that it is now possible to use NG double mutagenesis to try and locate origins of replication in yeast.

Lethal and mutagenic effects of nitrosoguanidine on synchronized Chlamydomonas

The lethal and mutagenic effects of 5 mug/ml N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were maximal during the nuclear S-period of synchronously grown Chlamydomonas reinhardtii. This was revealed by a 50% drop in survival and a 50- to 100-fold increase in the recovery of slow-growth mutants (up to 40% of the survivors) which were first recognized as small colonies on agar medium. Partial characterization of these isolates revealed about 50% to be stable on subculture, and several were demonstrated to be either acetate-dependent, dark-lethal (light-dependent), or acetate-sensitive mutants. There was no significant increase of lethality or of slow-growth mutants correlated with treatment during the chloroplast DNA replication phase of the cell-cycle. The results of genetic analysis with 13 mutants induced during the nuclear S-period were consistent with their nuclear origin. These analyses were hampered by the high proportion of lethality among the progeny of most crosses. It is concluded that the enhanced mutant induction among nuclear S-phase cells may indicate preferential mutagenesis of replication fork DNA and induction of multiple-closely-linked mutations, as in some bacteria. Consequently, for C. reinhardtii, caution should be exercised in drawing relationships between abnormal behavioral and biochemical phenotypes in MNNG-induced mutants.

Mechanism of nuclear DNA replication in radicles of germinating cotton

The mechanism of nuclear DNA replication in radicles of germinating cotton (Gossypium barbadense) was investigated. Pulse-labeling with [(3)H]thymidine indicates that replication intermediates are of small molecular weight (4-10S) and behave as if single-stranded. Prolonged labeling indicates that intermediates are of discrete size, suggesting a mechanism of discontinuous replication. Electron microscopy of nuclear DNA demonstrates a complex architecture which may include "eye forms" and "forks" similar to those reported in prokaryotes and animal systems. The possible universality of DNA replication mechanisms is discussed.