Vitamin C-mediated minimisation of Rogor-induced genotoxicity

The genotoxic effect of Rogor (an organophosphorous pesticide) at concentrations used in agriculture was studied in terms of mitotic index in onion root-tip cells, chromosome abnormalities and meiotic index in mice and lastly, lethal mutation rate in Drosophila. It was observed that the pesticide could (i) cause mitotic as well as meiotic inhibition, (ii) increase the clastogenicity rate and (iii) induce lethal mutations. The modificatory role, if any, of L-ascorbic acid (vitamin C) was studied after administering the vitamin concurrently with the pesticide. It was observed that the cytogenetic toxicity of Rogor can be appreciably minimised by vitamin C. The possible mode of antigenotoxic action of vitamin C was discussed.

The PRE4 gene codes for a subunit of the yeast proteasome necessary for peptidylglutamyl-peptide-hydrolyzing activity. Mutations link the proteasome to stress- and ubiquitin-dependent proteolysis

Proteinase yscE, the yeast proteasome, is a member of the nonlysosomal, high molecular mass (approximately 700 kDa) multifunctional proteinase complexes that are highly conserved from yeast to man. We have isolated mutants defective in one of the three proteolytic activities of the enzyme complex, i.e. in cleavage of peptide bonds after acidic amino acids. Using one of these mutants (pre4-1), we cloned the PRE4 gene and uncovered an open reading frame with 266 amino acids coding for a predicted protein of 29.4 kDa. The protein proved to be a subunit of proteinase yscE. The Pre4 amino acid sequence shows strong homology to the beta-subunit of the Xenopus laevis proteasome. Chromosomal deletion of the PRE4 gene is lethal. The pre4-1 mutant allele was cloned and sequenced. The mutant protein is shortened by 15 amino acids at the carboxyl terminus. Mutations (pre1-1, pre2-2) in the chymotrypsin-like activity of proteinase yscE uncovered the enzyme to be involved in ubiquitin-linked and stress-dependent proteolytic pathways. In contrast to these mutants, pre4-1 mutants did not exhibit any apparent stress-dependent phenotypes. However, pre1-1 pre4-1 double mutants showed enhanced canavanine sensitivity and increased accumulation of ubiquitin protein conjugates, as compared with pre1-1 single mutants.

[Genetic disorders in laboratory mice, exposed in the region of the Chernobyl Atomic Power Plant four years after the accident]

Mice (CBA x C57BL)F1, of both sex and males C57BL were exposed within the 10 km zone of the Chernobyl Nuclear Power Station during 14, 22 and 34 days. Genetic damages of chronic radiation in the exposed adult mice, and in the course of embryogenesis, was studied. The gonadal absorbed radiation doses of the exposed mice varied from 0.5 to 1.5 Gy. The frequencies of dominant lethal mutations, abnormal sperm heads and reciprocal translocations linearly increase with increasing absorbed radiation doses. Among 49 males exposed at the embryonal stage one male heterozygote for reciprocal translocation was revealed. In other males of this group, reciprocal translocation yield was low. No uniform dependence of genetic effects from mice genotype was revealed.

Phenotypes of lethal alleles of the recessive temperature sensitive paralytic mutant stambh A of Drosophila melanogaster suggest its neurogenic function

The mutant stambhA1 (2-56.8) of Drosophila melanogaster was identified as a reversible temperature sensitive adult and larval paralytic. We have (i) isolated and analysed phenotypes of one new homozygous viable paralytic allele and two recessive unconditional embryonic lethal alleles of stmA and (ii) studied the interaction of the viable paralytic alleles with ts paralytic mutants napts1 (2-55.2) and parats1 (1-53.9). The homozygous viable paralytic alleles stmA2 and stmA1 are semi dominant neomorphs. The lethal alleles stmA12 and stmA7 appear to be amorphs. Unhatched embryos expressing lethal stmA alleles showed hypotrophy of the anterior dorsal cuticle overlying the brain with a concomitant hypertrophy of the anterior dorsal neurogenic region (the brain). The ventral cuticle was poorly differentiated, and the ventral nerve chord showed mild hypertrophy and poor organisation. The epidermal cells in 12-13 h old embryos did not show the normal palisade layer arrangement. These phenotypes are similar to mutant phenotypes of the neurogenic class of genes whose wild type functions are necessary for intercellular communication. The alleles stmA1 and stmA2 do not appear to interact with the paralytic mutants napts1 or parats1 in double mutant combinations. On the basis of our results it is proposed that stmA may belong to the neurogenic class of genes.

Retinoblastoma protein and the cell cycle

Deregulation of the cell cycle may contribute one of the primary mechanisms through which cancer arises. Eukaryotic cell division has been found to be a strictly controlled process, involving response to both positive and negative external signals and assessment of the cell's internal state. Several recent discoveries have strengthened and refined the theory that the retinoblastoma protein is involved in the decision between cell division and differentiation, and have begun to provide an outline of the nature of this involvement.

Transgenic mouse model of X-linked cleft palate

A transgenic mouse line, PyLMP.5, exhibited a sex-linked lethality not observed in any other lines expressing the transgene. In this unique line, the transgene integrated into the X chromosome, yielding a simple tandem duplication of the insert sequences with minimal, if any, additional rearrangement of the cellular sequences. The predominant phenotype was a cleft secondary palate and neonatal lethality in males. Survival of females was dependent on the mouse strain background. The disrupted cellular sequences have been mapped to the proximal region of the mouse X chromosome. The disrupted locus may represent the mouse counterpart to a human locus mutated in an X-linked cleft secondary palate syndrome.

Hybrid lethal systems in the Drosophila melanogaster species complex. II. The Zygotic hybrid rescue (Zhr) gene of D. melanogaster

Hybrid females from Drosophila simulans females x Drosophila melanogaster males die as embryos while hybrid males from the reciprocal cross die as larvae. We have recovered a mutation in melanogaster that rescues the former hybrid females. It was located on the X chromosome at a position close to the centromere, and it was a zygotically acting gene, in contrast with mhr (maternal hybrid rescue) in simulans that rescues the same hybrids maternally. We named it Zhr (Zygotic hybrid rescue). The gene also rescues hybrid females from embryonic lethals in crosses of Drosophila mauritiana females x D. melanogaster males and of Drosophila sechellia females x D. melanogaster males. Independence of the hybrid embryonic lethality and the hybrid larval lethality suggested in a companion study was confirmed by employing two rescue genes, Zhr and Hmr (Hybrid male rescue), in doubly lethal hybrids. A model is proposed to explain the genetic mechanisms of hybrid lethalities as well as the evolutionary pathways.

Genetic analysis of the foraging microregion of Drosophila melanogaster

The rover/sitter polymorphism in Drosophila melanogaster larval behaviour is a unique example of a genetically determined, naturally occurring behavioural polymorphism. Allelic variation at the foraging locus (for) accounts for the rover (long foraging paths) and sitter (short foraging paths) phenotypes. We previously developed lethal tagging and used deficiency mapping to place for in the 24A3-C5 interval on the polytene chromosome map, thereby defining the for microregion. Here, we subjected this microregion to mutational analysis to (i) isolate putative lethal foraging mutations and characterize their behavioural phenotypes to assess whether or not for is a vital locus, (ii) generate cytologically detectable chromosome rearrangements with breakpoints in or near for for more precise localization and for future molecular analysis of the for gene, and (iii) identify other gene loci in the immediate vicinity of the for locus. We recovered 10 gamma-induced and 33 ethyl methanesulfonate (EMS) induced new mutations that define seven complementation groups in 24A3-D4. Two new EMS-induced lethal for alleles and four gamma-induced rearrangements with breakpoints in for were identified, which allowed us to further localize for to 24A3-5. All lethal mutations in for resulted in an altered behavioural phenotype providing evidence that both vital and behavioural functions are encoded by for.

A mobile group I intron from Physarum polycephalum can insert itself and induce point mutations in the nuclear ribosomal DNA of saccharomyces cerevisiae

Pp LSU3 is a mobile group I intron in the extrachromosomal nuclear ribosomal DNA (rDNA) of Physarum polycephalum. As found for other mobile introns, Pp LSU3 encodes a site-specific endonuclease, I-Ppo, which mediates "homing" to unoccupied target sites in Physarum rDNA. The recognition sequence for this enzyme is conserved in all eucaryotic nuclear rDNAs. We have introduced this intron into a heterologous species, Saccharomyces cerevisiae, in which nuclear group I introns have not been detected. The expression of Pp LSU3, under control of the inducible GAL10 promoter, was found to be lethal as a consequence of double-strand breaks in the rDNA. However, surviving colonies that are resistant to the lethal effects of I-Ppo because of alterations in the rDNA at the cleavage site were recovered readily. These survivors are of two classes. The first comprises cells that acquired one of three types of point mutations. The second comprises cells in which Pp LSU3 became inserted into the rDNA. In both cases, each resistant survivor appears to carry the same alterations in all approximately 150 rDNA repeats. When it is embedded in yeast rDNA, Pp LSU3 leads to the synthesis of I-Ppo and appears to be mobile in appropriate genetic crosses. The existence of yeast cells carrying a mobile intron should allow dissection of the steps that allow expression of the highly unusual I-Ppo gene.

Genetic instability in Drosophila melanogaster mediated by hobo transposable elements

Eight independent recessive lethal mutations that occurred on derivatives of an unstable X chromosome (Uc) in Drosophila melanogaster were analyzed by a combination of genetic and molecular techniques. Seven of the mutations were localized to complementation groups in polytene chromosome bands 6E; 7A. In situ hybridization and genomic Southern analysis established that hobo transposable elements were associated with all seven of the mutations. Six mutations involved deletions of DNA, some of which were large enough to be seen cytologically, and in each case, a hobo element was inserted at the junction of the deletion's breakpoints. A seventh mutation was associated with a small inversion between 6F and 7A-B and a hobo element was inserted at one of its breakpoints. One of the mutant chromosomes had an active hobo-mediated instability, manifested by the recurrent production of mutations of the carmine (cm) locus in bands 6E5-6. This instability persisted for many generations in several sublines of an inbred stock. Two levels of instability, high and basal, were distinguished. Sublines with high instability had two hobo elements in the 6E-F region and produced cm mutations by deleting the segment between the two hobos; a single hobo element remained at the junction of the deletion breakpoints. Sublines with low instability had only one hobo element in the 6E-F region, but they also produced deletion mutations of cm. Both types of sublines also acquired hobo-mediated inversions on the X chromosome. Collectively, these results suggest that interactions between hobo elements are responsible for the instability of Uc. It is proposed that interactions between widely separated elements produce gross rearrangements that restructure the chromosome and that interactions between nearby elements cause regional instabilities manifested by the recurrence of specific mutations. These regional instabilities may arise when a copy of hobo transposes a short distance, creating a pair of hobos that can interact to produce small rearrangements.

Studies on mutagen sensitive strains of Drosophila melanogaster. XI. Survival (dominant lethality) after X-irradiation and relation to recessive lethals and translocations

Muller-5 males of Drosophila melanogaster were irradiated in N2 or O2 and mated to excision repair deficient, post-replication repair deficient (mei-9a, mei-41D5, mus101D1, mus201D1, mus302D1, mus306D1 and mus308D2) or repair proficient females. The surviving fraction (dominant lethality) was estimated in the F1 and used to reassess existing recessive lethal and translocation data. The surviving fraction was found to decrease if repair deficient females were used (maternal effect). The dose-effect curves are often biphasic with a steeper slope at low doses than at high (> or = 5 Gy) doses of X-rays. The high dose part of the curve is sensitive to oxygenation during irradiation and is affected significantly by the mutants with low fertility (mei-9, mus101 and mus302). The low dose component is not sensitive to oxygenation during irradiation and seems influenced by all seven repair deficient mutants. The sensitivity of the high dose part to oxygenation suggests that this part is related mainly to DNA break damage, while in the low dose part base damage seems more important. Existing recessive lethal and translocation data were plotted against the surviving fraction for a reassessment. In excision repair deficient mutants translocation induction is lower compared to repair proficient flies at the same level of survival (i.e., dominant lethality). Likewise in post-replication repair deficient mutants induction of recessive lethals is decreased. However the frequency of respectively induced recessive lethals and translocations obtained at the same level of X-rays was the same in repair deficient and proficient backgrounds. It is concluded that genetic damage recovered in a repair deficient background is likely to be qualitatively different even if the frequency of the damage induced by a given dose is not altered.

The missing link in phage lysis of gram-positive bacteria: gene 14 of Bacillus subtilis phage phi 29 encodes the functional homolog of lambda S protein

In most bacteriophages of gram-negative bacteria, the phage endolysin is released to its murein substrate through a lesion in the inner membrane. The lesion is brought about by a second phage-encoded lysis function. For the first time, we present evidence that the same strategy is elaborated by a phage of a gram-positive bacterium. Thus, there appears to be an evolutionarily conserved lysis pathway for most phages whether their host bacterium is gram negative or gram positive. Phage phi 29 gene 14, the product of which is required for efficient lysis of Bacillus subtilis, was cloned in Escherichia coli. Production of protein 14 in E. coli resulted in cell death, whereas production of protein 14 concomitantly with the phi 29 lysozyme or unrelated murein-degrading enzymes led to lysis, suggesting that membrane-bound protein 14 induces a nonspecific lesion in the cytoplasmic membrane.

Preferential transposition of Drosophila P elements to nearby chromosomal sites

Two different schemes were used to demonstrate that Drosophila P elements preferentially transpose into genomic regions close to their starting sites. A starting element with weak rosy+ marker gene expression was mobilized from its location in the subtelomeric region of the 1,300-kb Dp1187 minichromosome. Among progeny lines with altered rosy+ expression, a much higher than expected frequency contained new insertions on Dp1187. Terminal deficiencies were also recovered frequently. In a second screen, a rosy(+)-marked element causing a lethal mutation of the cactus gene was mobilized in male and female germlines, and viable revertant chromosomes were recovered that still contained a rosy+ gene due to an intrachromosomal transposition. New transpositions recovered using both methods were mapped between 0 and 128 kb from the starting site. Our results suggested that some mechanism elevates the frequency 43-67-fold with which a P element inserts near its starting site. Local transposition is likely to be useful for enhancing the rate of insertional mutation within predetermined regions of the genome.

Efficient and dispersed local P element transposition from Drosophila females

We have investigated how Drosophila P element insertions are distributed in the chromosomal region near their starting site. A single P element residing in the euchromatin of minichromosome Dp1187 was mobilized following a cross to the delta 2-3 (99B) strain, and progeny bearing transpositions were identified with a minimum of bias by performing Southern blots on progeny. Approximately 1-2% of all progeny minichromosomes contained new insertions. Many of these "local transpositions" landed very close to or within the starting P element; however, nearly 1% of all progeny chromosomes contained new insertions 1-180 kb from the donor element. More local insertions were observed in the progeny of females than from male parents, and most occurred in a preferred orientation relative to the starting element. These observations suggested that donor elements are frequently excised and reinserted locally without ever dissociating from a transposition complex. The high frequency and diverse distribution of local transpositions recovered from females suggested that the efficiency of insertional mutagenesis can be significantly enhanced by using a starting P element(s) located near the target of interest.

Hybrid lethal systems in the Drosophila melanogaster species complex. I. The maternal hybrid rescue (mhr) gene of Drosophila simulans

Hybrid females from Drosophila simulans females x Drosophila melanogaster males die as embryos while hybrid males from the reciprocal cross die as late larvae. The other two classes are sterile adults. Letting C, X, and Y designate egg cytoplasm, X, and Y chromosomes, respectively, and subscripts m and s stand for melanogaster and simulans, CmXmYs males are lethal in the larval stage and are rescued by the previously reported genes, Lhr (Lethal hybrid rescue) in simulans or Hmr (Hybrid male rescue) in melanogaster. We report here another rescue gene located on the second chromosome of simulans, mhr (maternal hybrid rescue) that, when present in the mother, rescues CsXmXs females from embryonic lethality. It has been postulated that the hybrids not carrying the Xs like CmXmYs males are larval lethal and that the hybrids carrying both the Cs and the Xm like CsXmXs females are embryonic lethal. According to these postulates CsXmYs males (obtained by mating attached-X simulans females to melanogaster males) should be doubly lethal, at both embryo and larval stages. When both rescuing genes are present, Hmr in the father and mhr in the mother, males of this genotype are fully viable, as predicted.

Abnormal anaphase resolution (aar): a locus required for progression through mitosis in Drosophila

We describe a new mitotic locus of Drosophila melanogaster required for the progression through mitosis in the syncytial embryo and in late larval development. The locus aar (abnormal anaphase resolution) maps to the cytological interval 85E7-F16 and was identified by two alleles. The aar1 allele causes pupal lethality. Larval neuroblasts show an elevated mitotic index with high chromosome condensation and stretched and lagging chromatids during anaphase. aar2 produces fully viable but sterile females. aar1/aar2 females lay eggs that develop mitotic figures with similar abnormalities to those observed in neuroblasts. Indirect immunofluorescence of these embryos indicates that the centrosome cycle appears normal, although some abnormal spindle microtubules can be seen during mitosis.

More mog genes that influence the switch from spermatogenesis to oogenesis in the hermaphrodite germ line of Caenorhabditis elegans

The Caenorhabditis elegans XX animal possesses a hermaphrodite germ line, producing first sperm, then oocytes. In this paper, we report the genetic identification of five genes, mog-2, mog-3, mog-4, mog-5, and mog-6, that influence the hermaphrodite switch from spermatogenesis to oogenesis. In mog-2-mog-6 mutants, spermatogenesis continues past the time at which hermaphrodites normally switch into oogenesis and no oocytes are observed. Therefore, in these mutants, germ cells are transformed from a female fate (oocyte) to a male fate (sperm). The fem-3 gene is one of five genes that acts at the end of the germline sex determination pathway to direct spermatogenesis. Analyses of mog;fem-3 double mutants suggest that the mog-2-mog-6 genes act before fem-3; thus these genes may be in a position to negatively regulate fem-3 or one of the other terminal regulators of germline sex determination. Double mutants of fem-3 and any one of the mog mutations make oocytes. Using these double mutants, we show that oocytes from any mog;fem-3 double mutant are defective in their ability to support embryogenesis. This maternal effect lethality indicates that each of the mog genes is required for embryogenesis. The two defects in mog-2-mog-6 mutants are similar to those of mog-1: all six mog genes eliminate the sperm/oocyte switch in hermaphrodites and cause maternal effect lethality. We propose that the mog-2-mog-6 mutations identify genes that act with mog-1 to effect the sperm/oocyte switch. We further speculate that the mog-1-mog-6 mutations all interfere with translational controls of fem-3 and other maternal mRNAs.

Comparison of lethal mutations of Drosophila melanogaster with D. simulans when detected by the attached-X method

The purpose of this study was to evaluate the attached-X method compared with the standard Basc method, and, using this method, to find out whether the observed differences in genetic polymorphisms are related to differences in lethal mutation rates in D. melanogaster and D. simulans. When EMS-treated Drosophila melanogaster males are mated to untreated attached-X females, a decrease in the progeny sex ratio (male/female + male) is observed due to the induced lethal mutations on the X chromosome. The decrease in the frequency of male progeny were shown as the attached-X index. The expected male number is calculated from the control sex ratio. The difference between the expected and the observed male numbers, expressed as the ratio to the expected male number, defines the attached-X index. The index values for various EMS concentrations were compared to the lethal frequencies obtained by the standard Basc method for the same EMS treatments, and gave a highly positive correlation (gamma = 0.993, p < 0.01, d.f. = 2), thus providing an alternative method for evaluation of possible mutagens. The attached-X method was applied to D. simulans, of which natural populations are known to have relatively low genetic variation, and frequencies of the EMS-induced X chromosome lethal mutations were estimated and compared with those in D. melanogaster. The results indicate that D. melanogaster is slightly more sensitive in the sperm and spermatogonial stages, but less susceptible in the spermatid stage when compared with D. simulans.(ABSTRACT TRUNCATED AT 250 WORDS)

Two cysteine substitutions in procollagen I: a glycine replacement near the N-terminus of alpha 1(I) chain causes lethal osteogenesis imperfecta and a glycine replacement in the alpha 2(I) chain markedly destabilizes the triple helix

Cultured skin fibroblasts were examined from two probands with type II (lethal) osteogenesis imperfecta. One proband had a single base mutation which converted the glycine codon at position alpha 1-244 in the alpha 1(I) chain of procollagen I into a cysteine codon whereas the other had a similar mutation that converted the glycine codon at position alpha 2-787 of the alpha 2(I) chain into a cysteine codon. Both mutations produced post-translational overmodification of procollagen I. The Cys alpha 1-244 mutation, however, had a minimal effect on the thermal stability or secretion of the protein whereas the Cys alpha 2-787 mutation markedly decreased the thermal stability and, apparently as a result, essentially none of the mutated protein was secreted. The results provide clear exceptions to two previous generalizations about the position-specificity of glycine substitutions in procollagen I.

Genetic method for pre-classification of genotoxins into monofunctional or cross-linking agents

To characterize environmental carcinogens, there is a need to distinguish monofunctional genotoxic agents from those having cross-linking potential, because chemicals which can cross-link DNA are among the most potent carcinogens in rodents [Barbin and Bartsch, 1989] and humans [Allen et al., 1988; Kaldor et al., 1988]. Here we provide a genetic method for a pre-classification of genotoxins with respect to their functionality--monofunctional versus cross-linking. The procedure is based on the determination of relative clastogenic efficiency by a two-endpoint comparison in Drosophila: (i) induction of chromosome loss (CL), (ii) incidence of recessive lethal mutations (RL). Analysis of CL/RL ratios of 53 genotoxins, all mutagens in Drosophila, permitted distinction of 45 into two major categories: (i) 21 monofunctional agents with CL/RL indices generally < or = 1; (ii) 24 agents with ratios > 2 exhibiting DNA cross-linking properties. Within the group of monofunctional agents, CL/RL ratios tend to be low for SN1 agents, i.e., for N-ethyl-N-nitrosourea, N-ethyl-N'-nitro-N-nitrosoguanidine, and for N-nitrosodiethylamine. With cross-linking agents, the number of reactive groups appeared of minor importance as bi-, tri-, and tetrafunctional agents showed no significant differences in their CL/RL indices. Among 8 chemicals which could not be grouped into one of the two categories are two (adriamycin, daunomycin) regarded as intercalating agents. It is concluded that this two-endpoint analysis in Drosophila has prognostic value and can assist in the characterization of genotoxic agents with unknown mode of action.

A positive role in differentiation for the male doublesex protein of Drosophila

The doublesex (dsx) locus encodes male-specific and female-specific polypeptides that are essential for the proper differentiation of sexually dimorphic somatic features of Drosophila melanogaster. Ectopic expression of the male-specific dsx polypeptide was obtained by P-element-mediated transformation of flies with a construct bearing a fusion between the hsp70 heat shock promoter and dsx male-specific cDNA sequences. Heat shock-induced expression of the male cDNA in either sex resulted in three novel phenotypes: transformation of bristles on all legs toward a sex comb-like morphology, pigmentation of dorsal spinules and ventral setae in third-instar larvae, and lethality. These results were not predicted by previous models of dsx function, and provide evidence that the role of the male dsx protein includes activation of some aspects of male differentiation as well as repression of female differentiation.

Molecular genetics of aristaless, a prd-type homeo box gene involved in the morphogenesis of proximal and distal pattern elements in a subset of appendages in Drosophila

Viable aristaless (al) mutations of Drosophila affect pattern elements at both ends of the proximodistal axis in a subset of adult appendages. The al gene has been cloned and identified by P-element-mediated germ-line transformation with a genomic DNA fragment, which rescues a lethal mutation of al as well as aspects of the adult al phenotype. The al gene contains a prd-type homeo domain and a Pro/Gln-rich domain and, hence, probably encodes a transcription factor. Its transcript distribution in third-instar imaginal discs closely corresponds to the anlagen of the tissues that later become visibly affected in adult al mutants. The striking similarity of a bimodal al expression in different imaginal discs indicates that al is under the control of a "prepattern," which is shared at least among antennal, leg, and wing discs. The al gene is also transcribed during embryogenesis. Apart from a function in the ontogeny of specific larval head and tail organs, its embryonic transcript pattern suggests a possible role in early imaginal disc development.

The essential Escherichia coli msbA gene, a multicopy suppressor of null mutations in the htrB gene, is related to the universally conserved family of ATP-dependent translocators

We report the characterization of the msbA gene, isolated as a multicopy suppressor of the HtrB temperature-sensitive phenotype. The msbA gene maps to 20.5 min on the Escherichia coli genetic map and encodes a protein with an estimated molecular mass of 64,460 Da, with the properties of an integral membrane protein. The amino acid sequence of MsbA is very similar to those of the family of ATP-dependent translocators, which includes the haemolysin B protein of E. coli and the mammalian multidrug resistance (MDR) proteins. Mutational analysis of msbA indicates that it may form an operon with a downstream gene, orfE, and that both of these genes are essential for bacterial viability under all growth conditions tested.

N-ethyl-N-nitrosourea-induced prenatally lethal mutations define at least two complementation groups within the embryonic ectoderm development (eed) locus in mouse chromosome 7

Two loci [l(7)5Rn and l(7)6Rn] defined by N-ethyl-N-nitrosourea (ENU)-induced, prenatally lethal mutations were mapped by means of trans complementation crosses to mice carrying lethal deletions of the albino (c) locus in Chromosome (Chr) 7. Both loci were found to map to the subregion of the Mod-2-sh-1 interval that contains the eed (embryonic ectoderm development) locus, eed has been defined by the inability of embryos homozygous for certain c deletions to develop beyond the early stages of gastrulation. Evidence for at least two loci necessary for normal prenatal development, rather than one locus, that map within the eed interval came from the observation that two prenatally lethal mutations, 3354SB [l(7)5Rn3354SB] and 4234SB [l(7)6Rn4234SB], could complement each other in trans, but could not each be complemented individually by c deletions known to include the eed locus. A somewhat leaky allele of l(7)5Rn [l(7)5Rn1989SB] was also recovered, in which hemizygotes are often stillborn and homozygotes exhibit variable fitness and survival. The mapping of the loci defined by these mutations is likely to be useful for genetic, molecular, and phenotypic characterization of the eed region, and mutations at either locus (or both loci) may contribute to the eed phenotype.