Changing the p53 master regulatory network: ELEMENTary, my dear Mr Watson.

The p53 master regulatory network provides for the stress-responsive direct control of a vast number of genes in humans that can be grouped into several biological categories including cell-cycle control, apoptosis and DNA repair. Similar to other sequence-specific master regulators, there is a matrix of key components, which provide for variation within the p53 master regulatory network that include p53 itself, target response element sequences (REs) that provide for p53 regulation of target genes, chromatin, accessory proteins and transcription machinery. Changes in any of these can impact the expression of individual genes, groups of genes and the eventual biological responses. The many REs represent the core of the master regulatory network. Since defects or altered expression of p53 are associated with over 50% of all cancers and greater than 90% of p53 mutations are in the sequence-specific DNA-binding domain, it is important to understand the relationship between wild-type or mutant p53 proteins and the target response elements. In the words of the legendary detective Sherlock Holmes, it is 'Elementary, my dear Mr. Watson'.

Genes required for ionizing radiation resistance in yeast.

The ability of Saccharomyces cerevisiae to tolerate ionizing radiation damage requires many DNA-repair and checkpoint genes, most having human orthologs. A genome-wide screen of diploid mutants homozygous with respect to deletions of 3,670 nonessential genes revealed 107 new loci that influence gamma-ray sensitivity. Many affect replication, recombination and checkpoint functions. Nearly 90% were sensitive to other agents, and most new genes could be assigned to the following functional groups: chromatin remodeling, chromosome segregation, nuclear pore formation, transcription, Golgi/vacuolar activities, ubiquitin-mediated protein degradation, cytokinesis, mitochondrial activity and cell wall maintenance. Over 50% share homology with human genes, including 17 implicated in cancer, indicating that a large set of newly identified human genes may have related roles in the toleration of radiation damage.

In vivo site-directed mutagenesis using oligonucleotides.

Functional characterization of the genes of higher eukaryotes has been aided by their expression in model organisms and by analyzing site-specific changes in homologous genes in model systems such as the yeast Saccharomyces cerevisiae. Modifying sequences in yeast or other organisms such that no heterologous material is retained requires in vitro mutagenesis together with subcloning. PCR-based procedures that do not involve cloning are inefficient or require multistep reactions that increase the risk of additional mutations. An alternative approach, demonstrated in yeast, relies on transformation with an oligonucleotide, but the method is restricted to the generation of mutants with a selectable phenotype. Oligonucleotides, when combined with gap repair, have also been used to modify plasmids in yeast; however, this approach is limited by restriction-site availability. We have developed a mutagenesis approach in yeast based on transformation by unpurified oligonucleotides that allows the rapid creation of site-specific DNA mutations in vivo. A two-step, cloning-free process, referred to as delitto perfetto, generates products having only the desired mutation, such as a single or multiple base change, an insertion, a small or a large deletion, or even random mutations. The system provides for multiple rounds of mutation in a window up to 200 base pairs. The process is RAD52 dependent, is not constrained by the distribution of naturally occurring restriction sites, and requires minimal DNA sequencing. Because yeast is commonly used for random and selective cloning of genomic DNA from higher eukaryotes such as yeast artificial chromosomes, the delitto perfetto strategy also provides an efficient way to create precise changes in mammalian or other DNA sequences.

SIR functions are required for the toleration of an unrepaired double-strand break in a dispensable yeast chromosome.

Unrepaired DNA double-strand breaks (DSBs) typically result in G(2) arrest. Cell cycle progression can resume following repair of the DSBs or through adaptation to the checkpoint, even if the damage remains unrepaired. We developed a screen for factors in the yeast Saccharomyces cerevisiae that affect checkpoint control and/or viability in response to a single, unrepairable DSB that is induced by HO endonuclease in a dispensable yeast artificial chromosome containing human DNA. SIR2, -3, or -4 mutants exhibit a prolonged, RAD9-dependent G(2) arrest in response to the unrepairable DSB followed by a slow adaptation to the persistent break, leading to division and rearrest in the next G(2). There are a small number of additional cycles before permanent arrest as microcolonies. Thus, SIR genes, which repress silent mating type gene expression, are required for the adaptation and the prevention of indirect lethality resulting from an unrepairable DSB in nonessential DNA. Rapid adaptation to the G(2) checkpoint and high viability were restored in sir(-) strains containing additional deletions of the silent mating type loci HML and HMR, suggesting that genes under mating type control can reduce the toleration of a single DSB. However, coexpression of MATa1 and MATalpha2 in Sir(+) haploid cells did not lead to lethality from the HO-induced DSB, suggesting that toleration of an unrepaired DSB requires more than one Sir(+) function.

p53 mutants can often transactivate promoters containing a p21 but not Bax or PIG3 responsive elements.

The human p53 protein acts mainly as a stress inducible transcription factor transactivating several genes involved in cell cycle arrest (e.g. p21) or apoptosis (e.g. Bax, PIG3). Roughly half of all human tumours contains p53 missense mutations. Virtually all tumour-derived p53 mutants are unable to activate Bax transcription but some retain the ability to activate p21 transcription. Identification of these mutants may have valuable clinical implications. We have determined the transactivation ability of 77 p53 mutants using reporter yeast strains containing a p53-regulated ADE2 gene whose promoter is regulated by p53 responsive elements derived from the regulatory region of the p21, Bax and PIG3 genes. We also assessed the influence of temperature on transactivation. Our results indicate that a significant proportion of mutants [16/77 (21%); 10/64 (16%) considering only tumour-derived mutants] are transcriptionally active, especially with the p21 promoter. Discriminant mutants preferentially affect less conserved (P<0.04, Fisher's exact test), more rarely mutated (P<0.006, Fisher's exact test) amino acids. Temperature sensitivity is frequently observed, but is more common among discriminant than non-discriminant mutants (P<0.003, Fisher's exact test). Finally, we extended the analysis to a group of mutants isolated in BRCA-associated tumours that surprisingly were indistinguishable from wild type in standard transcription, growth suppression and apoptosis assays in human cells, but showed gain of function in transformation assays. The incidence of transcriptionally active mutations among this group was significantly higher than in the panel of mutants studied previously (P<0.001, Fisher's exact test). Since it is not possible to predict the behaviour of a mutant from first principles, we propose that the yeast assay be used to compile a functional p53 database and fill the gap between the biophysical, pharmacological and clinical fields.

Novel human p53 mutations that are toxic to yeast can enhance transactivation of specific promoters and reactivate tumor p53 mutants.

Since highly expressed human p53 can inhibit human and yeast cell growth, we predicted that p53 mutants could be generated with increased growth inhibition of the yeast Saccharomyces cerevisiae and that these would be useful for characterizing p53 functions and tumor p53 mutants. A random mutagenesis screen led to the isolation of mutations in the DNA binding domain that result in p53 being lethal even at moderate expression levels in yeast. Three independent mutants had an alanine change at the evolutionary invariant V122 in the L1 loop. The other toxic mutations affected codons 277 (C277R, C277W) and 279 (G279R). This latter amino acid change was also reported in tumors, while all the other mutations are novel. A recently developed rheostatable GALI promoter system that provides graded increases in expression of p53 was used to examine the transactivation function of the toxic mutations when expression was greatly reduced and cells were viable. At low expression levels the toxic mutants lacked transactivation from a 3xRGC responsive element (RE). Surprisingly some exhibited enhanced transactivation with p21 and bax REs. The V122A mutant was able to re-activate transactivation of various p53 tumor mutants and retained growth inhibition when co-expressed with dominant-negative tumor mutations. Upon expression in human Saos-2 cells the V122A p53 mutant caused growth suppression, was capable of transactivation and exhibited higher than wild type activity with the bax promoter in luciferase assays. A non-functional p53 tumor mutant was partially reactivated by V122A for both transactivation and growth suppression. Thus, the screen for toxic p53 mutants in yeast can identify novel p53 variants that may be useful in dissecting p53 regulated cellular responses and in developing p53-based cancer therapies.

The 3'-->5' exonuclease of DNA polymerase delta can substitute for the 5' flap endonuclease Rad27/Fen1 in processing Okazaki fragments and preventing genome instability.

Many DNA polymerases (Pol) have an intrinsic 3'-->5' exonuclease (Exo) activity which corrects polymerase errors and prevents mutations. We describe a role of the 3'-->5' Exo of Pol delta as a supplement or backup for the Rad27/Fen1 5' flap endonuclease. A yeast rad27 null allele was lethal in combination with Pol delta mutations in Exo I, Exo II, and Exo III motifs that inactivate its exonuclease, but it was viable with mutations in other parts of Pol delta. The rad27-p allele, which has little phenotypic effect by itself, was also lethal in combination with mutations in the Pol delta Exo I and Exo II motifs. However, rad27-p Pol delta Exo III double mutants were viable. They exhibited strong synergistic increases in CAN1 duplication mutations, intrachromosomal and interchromosomal recombination, and required the wild-type double-strand break repair genes RAD50, RAD51, and RAD52 for viability. Observed effects were similar to those of the rad27-null mutant deficient in the removal of 5' flaps in the lagging strand. These results suggest that the 3'-->5' Exo activity of Pol delta is redundant with Rad27/Fen1 for creating ligatable nicks between adjacent Okazaki fragments, possibly by reducing the amount of strand-displacement in the lagging strand.

p53 mutants exhibiting enhanced transcriptional activation and altered promoter selectivity are revealed using a sensitive, yeast-based functional assay.

Changes in promoter specificity and binding affinity that may be associated with p53 mutations or post-translational modifications are useful in understanding p53 structure/function relationships and categorizing tumor mutations. We have exploited variable expression of human p53 in yeast to identify mutants with novel phenotypes that would correspond to altered promoter selectivity and affinity. The p53 cDNA regions coding for the DNA binding and tetramerization domains were subjected to random PCR mutagenesis and were cloned directly by recombination in yeast into a vector with a GAL1 promoter whose level of expression could be easily varied. p53 variants exhibiting higher than wild type levels of transactivation (supertrans) for the RGC responsive element were identified at low level of p53 protein expression. All the p53 mutants obtained with this screen were located in the DNA binding domain. Two out of 17 supertrans mutants have been found in tumors. Six mutations were in the L1 loop region between amino acids 115 and 124. The transactivation potential of a panel of supertrans p53 mutants on different promoters was evaluated using the p53 responsive elements, RGC, PIG3, p21 and bax. Although all mutants retained some activity with all promoters, we found different patterns of induction based on strength and promoter specificity. In particular none of the mutants was supertrans for the p21 responsive element. Interestingly, further analysis in yeast showed that the transactivation function could be retained even in the presence of dominant-negative p53 tumor mutations that could inhibit wild type p53. Five mutants were also characterized in human cells in terms of growth suppression and transactivation of various promoters. These novel supertrans p53 mutants may be useful in studies aimed at dissecting p53 downstream pathways, understanding specific interactions between p53 and the DNA, and could replace wild type p53 in cancer gene therapy protocols. The approach may also prove useful in identifying p53 tumor mutations.

Biased distribution of inverted and direct Alus in the human genome: implications for insertion, exclusion, and genome stability.

Alu sequences, the most abundant class of large dispersed DNA repeats in human chromosomes, contribute to human genome dynamics. Recently we reported that long inverted repeats, including human Alus, can be strong initiators of genetic change in yeast. We proposed that the potential for interactions between adjacent, closely related Alus would influence their stability and this would be reflected in their distribution. We have undertaken an extensive computational analysis of all Alus (the database is at http://dir.niehs.nih.gov/ALU) to better understand their distribution and circumstances under which Alu sequences might affect genome stability. Alus separated by <650 bp were categorized according to orientation, length of regions sharing high sequence identity, distance between highly identical regions, and extent of sequence identity. Nearly 50% of all Alu pairs have long alignable regions (>275 bp), corresponding to nearly full-length Alus, regardless of orientation. There are dramatic differences in the distributions and character of Alu pairs with closely spaced, nearly identical regions. For Alu pairs that are directly repetitive, approximately 30% have highly identical regions separated by <20 bp, but only when the alignments correspond to near full-size or half-size Alus. The opposite is found for the distribution of inverted repeats: Alu pairs with aligned regions separated by <20 bp are rare. Furthermore, closely spaced direct and inverted Alus differ in their truncation patterns, suggesting differences in the mechanisms of insertion. At larger distances, the direct and inverted Alu pairs have similar distributions. We propose that sequence identity, orientation, and distance are important factors determining insertion of adjacent Alus, the frequency and spectrum of Alu-associated changes in the genome, and the contribution of Alu pairs to genome instability. Based on results in model systems and the present analysis, closely spaced inverted Alu pairs with long regions of alignment are likely at-risk motifs (ARMs) for genome instability.

Transformation-associated recombination (TAR) cloning of tumor-inducing Xmrk2 gene from Xiphophorus maculatus.

We modified the TAR methodology of YAC clone construction for application to fish genomic DNA isolated from Xiphophorus maculatus. YAC libraries were developed using the XIR1 repeat sequence as the recombinational hook. Construction of these libraries demonstrates that Xiphophorus DNA sequences can function as hooks in the yeast recombination system and that X. maculatus genomic DNA contains sequences that provide origin of replication function in yeast. By screening a subset of Xiphophorus YAC clones, we isolated a clone harboring the Xmrk2 locus that is associated with spontaneous and induced melanomagenesis. Modifications of the TAR technique allowed the targeted cloning of specific genes from genomic regions ranging in size from cDNAs to several hundred kilobases. Specific genomic regions can be isolated in a directional manner from fixed map locations to saturate these areas with physical markers. We discuss the applications of these and other yeast recombinational processes to fish genetics.

A DNA polymerase epsilon mutant that specifically causes +1 frameshift mutations within homonucleotide runs in yeast.

The DNA polymerases delta and epsilon are the major replicative polymerases in the yeast Saccharomyces cerevisiae that possess 3' --> 5' exonuclease proofreading activity. Many errors arising during replication are corrected by these exonuclease activities. We have investigated the contributions of regions of Polepsilon other than the proofreading motifs to replication accuracy. An allele, pol2-C1089Y, was identified in a screen of Polepsilon mutants that in combination with an exonuclease I (exo1) mutation could cause a synergistic increase in mutations within homonucleotide runs. In contrast to other polymerase mutators, this allele specifically results in insertion frameshifts. When pol2-C1089Y was combined with deletions of EXO1 or RAD27 (homologue of human FEN1), mutation rates were increased for +1 frameshifts while there was almost no effect on -1 frameshifts. On the basis of genetic analysis, the pol2-C1089Y mutation did not cause a defect in proofreading. In combination with a deletion of the mismatch repair gene MSH2, the +1 frameshift mutation rate for a short homonucleotide run was increased nearly 100-fold whereas the -1 frameshift rate was unchanged. This suggests that the Pol2-C1089Y protein makes +1 frameshift errors during replication of homonucleotide runs and that these errors can be corrected by either mismatch repair (MMR) or proofreading (in short runs). This is the first report of a +1-specific mutator for homonucleotide runs in vivo. The pol2-C1089Y mutation defines a functionally important residue in Polepsilon.

Tying up loose ends: nonhomologous end-joining in Saccharomyces cerevisiae.

The ends of chromosomal DNA double-strand breaks (DSBs) can be accurately rejoined by at least two discrete pathways, homologous recombination and nonhomologous end-joining (NHEJ). The NHEJ pathway is essential for repair of specific classes of DSB termini in cells of the budding yeast Saccharomyces cerevisiae. Endonuclease-induced DSBs retaining complementary single-stranded DNA overhangs are repaired efficiently by end-joining. In contrast, damaged DSB ends (e.g., termini produced by ionizing radiation) are poor substrates for this pathway. NHEJ repair involves the functions of at least 10 genes, including YKU70, YKU80, DNL4, LIF1, SIR2, SIR3, SIR4, RAD50, MRE11, and XRS2. Most or all of these genes are required for efficient recombination-independent recircularization of linearized plasmids and for rejoining of EcoRI endonuclease-induced chromosomal DSBs in vivo. Several NHEJ mutants also display aberrant processing and rejoining of DSBs that are generated by HO endonuclease or formed spontaneously in dicentric plasmids. In addition, all NHEJ genes except DNL4 and LIF1 are required for stabilization of telomeric repeat sequences. Each of the proteins involved in NHEJ appears to bind, directly or through protein associations, with the ends of linear DNA. Enzymatic and/or structural roles in the rejoining of DSB termini have been postulated for several proteins within the group. Most yeast NHEJ genes have homologues in human cells and many biochemical activities and protein:protein interactions have been conserved in higher eucaryotes. Similarities and differences between NHEJ repair in yeast and mammalian cells are discussed.

Inverted Alu repeats unstable in yeast are excluded from the human genome.

The nearly one million ALU: repeats in human chromosomes are a potential threat to genome integrity. ALU:s form dense clusters where they frequently appear as inverted repeats, a sequence motif known to cause DNA rearrangements in model organisms. Using a yeast recombination system, we found that inverted ALU: pairs can be strong initiators of genetic instability. The highly recombinagenic potential of inverted ALU: pairs was dependent on the distance between the repeats and the level of sequence divergence. Even inverted ALU:s that were 86% homologous could efficiently stimulate recombination when separated by <20 bp. This stimulation was independent of mismatch repair. Mutations in the DNA metabolic genes RAD27 (FEN1), POL3 (polymerase delta) and MMS19 destabilized widely separated and diverged inverted ALU:s. Having defined factors affecting inverted ALU: repeat stability in yeast, we analyzed the distribution of ALU: pairs in the human genome. Closely spaced, highly homologous inverted ALU:s are rare, suggesting that they are unstable in humans. ALU: pairs were identified that are potential sites of genetic change.

Radial transformation-associated recombination cloning from the mouse genome: isolation of Tg.AC transgene with flanking DNAs.

Transformation-associated recombination (TAR) cloning allows entire genes and large chromosomal regions to be specifically, accurately, and quickly isolated from total genomic DNA. We report the first example of radial TAR cloning from the mouse genome. Tg.AC mice carry a zeta-globin promoter/v-Ha-ras transgene. Fluorescence in situ hybridization localized the transgene integrant as a single site proximal to the centromere of chromosome 11. Radial TAR cloning in yeast was utilized to create orientation-specific yeast artificial chromosomes (YACs) to explore the possibility that cis-flanking regions were involved in transgene expression. YACs containing variable lengths of 5' or 3' flanking chromosome 11 DNA and the Tg.AC transgene were specifically chosen, converted to bacterial artificial chromosomes (BACs), and assayed for their ability to promote transcription of the transgene following transfection into an FVB/N carcinoma cell line. A transgene-specific reverse transcription-polymerase chain reaction assay was utilized to examine RNA transcripts from stably transfected clones. All Tg.AC BACs expressed the transgene in this in vitro system. This report describes the cloning of the v-Ha-ras transgene and suggests that transcriptional activity may not require cis elements flanking the transgene's integration site.

Long inverted repeats are an at-risk motif for recombination in mammalian cells.

Certain DNA sequence motifs and structures can promote genomic instability. We have explored instability induced in mouse cells by long inverted repeats (LIRs). A cassette was constructed containing a herpes simplex virus thymidine kinase (tk) gene into which was inserted an LIR composed of two inverted copies of a 1.1-kb yeast URA3 gene sequence separated by a 200-bp spacer sequence. The tk gene was introduced into the genome of mouse Ltk(-) fibroblasts either by itself or in conjunction with a closely linked tk gene that was disrupted by an 8-bp XhoI linker insertion; rates of intrachromosomal homologous recombination between the markers were determined. Recombination between the two tk alleles was stimulated 5-fold by the LIR, as compared to a long direct repeat (LDR) insert, resulting in nearly 10(-5) events per cell per generation. Of the tk(+) segregants recovered from LIR-containing cell lines, 14% arose from gene conversions that eliminated the LIR, as compared to 3% of the tk(+) segregants from LDR cell lines, corresponding to a >20-fold increase in deletions at the LIR hotspot. Thus, an LIR, which is a common motif in mammalian genomes, is at risk for the stimulation of homologous recombination and possibly other genetic rearrangements.

Functional analysis of human FEN1 in Saccharomyces cerevisiae and its role in genome stability.

The flap endonuclease, FEN1, is an evolutionarily conserved component of DNA replication from archaebacteria to humans. Based on in vitro results, it processes Okazaki fragments during replication and is involved in base excision repair. FEN1 removes the last primer ribonucleotide on the lagging strand and it cleaves a 5' flap that may result from strand displacement during replication or during base excision repair. Its biological importance has been revealed largely through studies in the yeast Saccharomyces cerevisiae where deletion of the homologous gene RAD27 results in genome instability and mutagen sensitivity. While the in vivo function of Rad27 has been well characterized through genetic and biochemical approaches, little is understood about the in vivo functions of human FEN1. Guided by our recent results with yeast RAD27, we explored the function of human FEN1 in yeast. We found that the human FEN1 protein complements a yeast rad27 null mutant for a variety of defects including mutagen sensitivity, genetic instability and the synthetic lethal interactions of a rad27 rad51 and a rad27 pol3-01 mutant. Furthermore, a mutant form of FEN1 lacking nuclease function exhibits dominant-negative effects on cell growth and genome instability similar to those seen with the homologous yeast rad27 mutation. This genetic impact is stronger when the human and yeast PCNA-binding domains are exchanged. These data indicate that the human FEN1 and yeast Rad27 proteins act on the same substrate in vivo. Our study defines a sensitive yeast system for the identification and characterization of mutations in FEN1.

Repair of endonuclease-induced double-strand breaks in Saccharomyces cerevisiae: essential role for genes associated with nonhomologous end-joining.

Repair of double-strand breaks (DSBs) in chromosomal DNA by nonhomologous end-joining (NHEJ) is not well characterized in the yeast Saccharomyces cerevisiae. Here we demonstrate that several genes associated with NHEJ perform essential functions in the repair of endonuclease-induced DSBs in vivo. Galactose-induced expression of EcoRI endonuclease in rad50, mre11, or xrs2 mutants, which are deficient in plasmid DSB end-joining and some forms of recombination, resulted in G2 arrest and rapid cell killing. Endonuclease synthesis also produced moderate cell killing in sir4 strains. In contrast, EcoRI caused prolonged cell-cycle arrest of recombination-defective rad51, rad52, rad54, rad55, and rad57 mutants, but cells remained viable. Cell-cycle progression was inhibited in excision repair-defective rad1 mutants, but not in rad2 cells, indicating a role for Rad1 processing of the DSB ends. Phenotypic responses of additional mutants, including exo1, srs2, rad5, and rdh54 strains, suggest roles in recombinational repair, but not in NHEJ. Interestingly, the rapid cell killing in haploid rad50 and mre11 strains was largely eliminated in diploids, suggesting that the cohesive-ended DSBs could be efficiently repaired by homologous recombination throughout the cell cycle in the diploid mutants. These results demonstrate essential but separable roles for NHEJ pathway genes in the repair of chromosomal DSBs that are structurally similar to those occurring during cellular development.

A novel role in DNA metabolism for the binding of Fen1/Rad27 to PCNA and implications for genetic risk.

Fen1/Rad27 nuclease activity, which is important in DNA metabolism, is stimulated by proliferating cell nuclear antigen (PCNA) in vitro. The in vivo role of the PCNA interaction was investigated in the yeast Rad27. A nuclease-defective rad27 mutation had a dominant-negative effect that was suppressed by a mutation in the PCNA binding site, thereby demonstrating the importance of the Rad27-PCNA interaction. The PCNA-binding defect alone had little effect on mutation, recombination, and the methyl methanesulfonate (MMS) response in repair-competent cells, but it greatly amplified the MMS sensitivity of a rad51 mutant. Furthermore, the PCNA binding mutation resulted in lethality when combined with a homozygous or even a heterozygous pol3-01 mutation in the 3'-->5' exonuclease domain of DNA polymerase delta. These results suggest that phenotypically mild polymorphisms in DNA metabolic proteins can have dramatic consequences when combined.

Genetic factors affecting the impact of DNA polymerase delta proofreading activity on mutation avoidance in yeast.

Base selectivity, proofreading, and postreplication mismatch repair are important for replication fidelity. Because proofreading plays an important role in error correction, we have investigated factors that influence its impact in the yeast Saccharomyces cerevisiae. We have utilized a sensitive mutation detection system based on homonucleotide runs of 4 to 14 bases to examine the impact of DNA polymerase delta proofreading on mutation avoidance. The contribution of DNA polymerase delta proofreading on error avoidance was found to be similar to that of DNA polymerase epsilon proofreading in short homonucleotide runs (A4 and A5) but much greater than the contribution of DNA polymerase epsilon proofreading in longer runs. We have identified an intraprotein interaction affecting mutation prevention that results from mutations in the replication and the proofreading regions, resulting in an antimutator phenotype relative to a proofreading defect. Finally, a diploid strain with a defect in DNA polymerase delta proofreading exhibits a higher mutation rate than a haploid strain. We suggest that in the diploid population of proofreading defective cells there exists a transiently hypermutable fraction that would be inviable if cells were haploids.