The location of the feedback-specific region with the pyrimidine-3 locus of Neurospora crassa.

Summary Thepyrimidine-3 locus ofNeurospora crassa specifies two enzyme activities, pyrimidine-specific carbamyl phosphate synthetase (CPSpyr) and aspartate transcarbamylase (ATC). ATC is translationally distal. CPSpyr, but not ATC, is subject to feedback inhibition by uridine triphosphate (UTP). To investigate the location of the feedback-specific region within the locus, inhibition of a number ofpyr-3 alleles by UTP was investigated. All CPS⁺ ATC^- polar alleles, revertants of CPS^- ATC^- polar alleles, and 5-fluorouracil-resistant mutants had normal UTP response. The location of the feedback-specific region is in or close to the CPS-specific region.Supported by Science Research Council Grant B/RG/2981     

Morphogenesis in pinoid mutants of Arabidopsis thaliana

A series of mutants of Arabidopsis thaliana was selected in which the inflorescence stem elongates but loses the ability to produce flower primordia on its flanks. Mutants fell into two classes, further occurrences of pin-formed mutants and mutations at a new locus named pinoid. As well as causing inflorescence defects, pinoid mutations result in pleiotropic defects in the development of floral organs, cotyledons and leaves. Most changes involve the number of organs produced rather than their differentiation suggesting that PINOID controls an early general step in meristem development. pinoid mutant defects are similar to those seen in pin-formed mutants for inflorescences and flowers, but different for cotyledons and leaves indicating that the two genes have separate but overlapping functions. A defect in polar auxin transport is implicated in the pin-formed mutant phenotype, but in young inflorescence stems of even the strongest pinoid mutants it occurs at close to wild-type levels. It is markedly reduced only after stems have ceased elongating. Thus, it is likely that polar auxin transport is secondarily affected in pinoid mutants rather than being directly controlled by the PINOID gene product. Even so, double mutant studies indicate that the process controlled by PINOID overlaps with that specified by the AUXIN RESISTANT1 gene, suggesting that PINOID plays some role in an auxin-related process.     

Mutations at the Asc locus of tomato confer resistance to the fungal pathogen Alternaria alternata f. sp. lycopersici

The fungal pathogen Alternaria alternata f. sp. lycopersici produces host-selective AAL-toxins that cause Alternaria stem canker in tomato. Susceptibility to the disease is based on the relative sensitivity of the host to the AAL-toxins and is controlled by the Asc locus on chromosome 3L. Chemical mutagenesis was employed to study the genetic basis of sensitivity to AAL-toxins and susceptibility to fungal infection. Following the treatment of seeds of a susceptible line with ethyl methanesulphonate (EMS), resistant M₂ mutants were obtained. Most plants with induced resistances showed toxin-sensitivity responses that were comparable to those of resistant control lines carrying the Asc locus. In addition, genetic analysis of the mutagenised plants indicated that the mutations occurred at the Asc locus. Furthermore, novel mutants were identified that were insensitive to the AAL-toxins at the seedling stage but toxin-sensitive and susceptible to fungal infection at mature stages. No AAL-toxin-insensitive insertion mutants were identified following a transposon mutagenesis procedure. Molecular mechanisms involved in host defence against A a. lycopersici are discussed.     

Map location of arginyl-tRNA synthetase mutations in Escherichia coli K-12

Summary Mutants of Escherichia coli K-12 previously isolated in the authors' laboratory have reduced arginyl-tRNA synthetase activity. The mutants fall into two classes. All mutants grow slowly on arginine-free medium. On arginine-supplemented medium some mutants grow at a normal rate (Class I) while others still grow slowly (Class II). Matings were performed to located a Class I and a Class II mutation on the E. coli chromosome map, and on the basis of our results we have assigned both to one locus, argS.     

A genetic and physiological analysis of late flowering mutants in Arabidopsis thaliana

Summary Monogenic mutants of the early ecotype Landsberg erecta were selected on the basis of late flowering under long day (LD) conditions after treatment with ethyl methanesulphonate or irradiation. In addition to later flowering the number of rosette and cauline leaves is proportionally higher in all mutants, although the correlation coefficient between the two parameters is not the same for all genotypes. Forty-two independently induced mutants were found to represent mutations at 11 loci. The mutations were either recessive, intermediate (co locus) or almost completely dominant (fwa locus). The loci are located at distinct positions on four of the five Arabidopsis chromosomes. Recombinants carrying mutations at different loci flower later than or as late as the later parental mutant. This distinction led to the assignment of eight of the loci to three epistatic groups. In wild type, vernalization promotes flowering to a small extent. For mutants at the loci fca, fve, fy and fpa, vernalization has a large effect both under LD and short day (SD) conditions, whereas co, gi, fd and fwa mutants are almost completely insensitive to this treatment. SD induces later flowering except for mutants at the co and gi loci, which flower with the same number of leaves under LD and SD conditions. This differential response of the mutants to environmental factors and their subdivision into epistatic groups is discussed in relation to a causal model for floral initiation in Arabidopsis thaliana.     

Polar assembly and scaffolding proteins of the virulence-associated ESX-1 secretory apparatus in mycobacteria

The ESX‐1 secretion system is required for pathogenicity of Mycobacterium tuberculosis (Mtb). Despite considerable research, little is known about the structural components of ESX‐1, or how these proteins are assembled into the active secretion apparatus. Here, we exploit the functionally related ESX‐1 apparatus of Mycobacterium smegmatis (Ms) to show that fluorescently tagged proteins required for ESX‐1 activity consistently localize to the cell pole, identified by time‐lapse fluoro‐microscopy as the non‐septal (old) pole. Deletions in Msesx1 prevented polar localization of tagged proteins, indicating the need for specific protein–protein interactions in polar trafficking. Remarkably, expression of the Mtbesx1 locus in Msesx1 mutants restored polar localization of tagged proteins, indicating establishment of the MtbESX‐1 apparatus in M. smegmatis. This observation illustrates the cross‐species conservation of protein interactions governing assembly of ESX‐1, as well as polar localization. Importantly, we describe novel non‐esx1‐encoded proteins, which affect ESX‐1 activity, which colocalize with ESX‐1, and which are required for ESX‐1 recruitment and assembly. This analysis provides new insights into the molecular assembly of this important determinant of Mtb virulence.     

A cytogenetic analysis of the lozenge pseudoalleles in drosophila

Summary 1. A cytogenetic analysis of 18 independent lz mutants is presented.2. On a phenotypic basis the mutants can be separated into three classes.3. The results of a recombination analysis show that each of the 18 mutants can be assigned to one of three loci. No case where a lz mutant involved more than one locus was found.4. Both phenotypic and recombination evidence place the amx mutant outside the lz pseudoallelic system.5. Cytologically the lz pseudoalleles have been localized to section 8D of the salivary X chromosome.6. The relevance of the data presented to the nature of pseudoallelism is discussed.     

Instability at the y-1 locus of Chlamydomonas reinhardtii

Summary Twenty-three spontaneous yellow mutants were isolated from two stable green strains of the unicellular green alga Chlamydomonas reinhardtii. Genetic characterization indicated that 22 of 23 mutants had a mutation at the y-1 locus, and all 22 y-1 alleles were unstable. Crosses designed to follow the inheritance of instability at the y-1 locus showed that instability is caused by a single genetic factor located at the y-1 locus or very close to it.     

Activities of auxin polar transport in inflorescence axes of flower mutants ofArabidopsis thaliana: Relevance to flower formation and growth

Relationships between the activity of auxin polar transport and flower formation were studied using several flower mutants ofArabidopsis thaliana. The activity of auxin polar transport in the upper portion of inflorescence axis of wildtype plants ofArabidopsis thaliana was significantly lower than that of the basal part. The activities of auxin polar transport in the upper portion of inflorescence axes ofap1 andclv1 mutants were significantly higher than that of wild-type plant. However, those of other flower mutants tested,ap3-1, ag, pi, Fl-40, Fl-54, Fl-89 andpin-formed, were extremely low as compared with that of wild one. We got some evidence that the reduction of the activity of auxin polar transport is concerned with the growth and development of plants. We could mimic it by the removal of all flowers and pods including mature or immature seeds. Moreover, artificial pollination inap3-1 andpi mutants, in which no seeds are found naturally, resulted in the partial recovery of the activity of auxin polar transport in inflorescence axis. Considering these results in this study together with the fact that inhibitors of auxin polar transport generated almost same disruptions ofpin-formed orpinoid mutants which normally had no flowers in inflorescence axis (Okadaet al. 1991, Uedaet al. 1992, Bennettet al. 1995), the systern of auxin polar transport and its activity in inflorescence axis seems to be essential for the development of flower bud in early stage ofArabidopsis thaliana, and the activity of auxin polar transport is also regulated by the formation of flowers and seeds in inflorescence axis.     

Characterization of three hormone mutants of Nicotiana plumbaginifolia: evidence for a common ABA deficiency

Summary Various auxin-resistant Nicotiana plumbaginifolia mutants have already been isolated, including 1217 which shows cross-resistance to paclobutrazol. Recently, a cytokinin-resistant mutant, CKR1, has been characterized and has been shown to be affected in abscisic acid (ABA) biosynthesis. We have isolated a new mutant, Esg152, which was selected on the basis of its early germination. In each of these mutants, resistance is due to a recessive nuclear mutation at a single locus. Complementation analysis indicated that mutants I217, CKR1 and Esg152 belong to the same complementation group. They have a similar phenotype, which includes a reduction in seed dormancy and an increased tendency to wilt. These mutants display an increased auxin tolerance and enhanced root formation when leaf or hypocotyl sections are cultivated on auxin. By immunoenzymatic methods, we show that the endogenous levels of ABA are significantly lower than in the wild-type. We have assigned the symbol aba1 to the recessive alleles of the locus affected in the three mutants. The complexity of hormonal interactions is discussed briefly emerging from a consideration of this class of mutants.     

Polarised complementation at the pyrimidine-3 locus of Neurospora

Summary A re-evaluation of complementation at the pyrimidine-3 locus ofNeurospora suggests the existence of polarity. This apparent polarity is supported by polar complementation of the majority of a number of mutants induced with the acridine ICR-170, and confirmed by the behavior of an allele which is shown by reversion studies to be a frameshift mutant. Supported by Public Health Service Grant AI-01462.     

Nucleoside auxotrophy in Drosophila: An autosomal locus yielding mutants supplementable by purine and pyrimidine ribonucleosides

Summary Two allelic auxotrophic mutants at a locus close to the bw locus (2–104.5) of Drosophila melanogaster are described. The mutants respond to dietary ribonucleosides (uridine, cytidine, adenosine, guanosine and inosine) but less well to bases or pyrimidine precursors. This phenotype is unique to these mutants.We suggest that the mutants are defective in phosphoribosyl pyrophosphate biosynthesis.     

Response of ad-2 mutants of Saccharomyces cerevisiae to carbon dioxide

Summary Confirmation that the ad-2 locus of yeast controls the carboxylation of aminoimidazole ribonucleotide (AIR) to 5-amino-4-imidazole carboxylate ribonucleotide (CAIR) is provided by the observation that 21 out of a sample of 113 ad-2 mutants were affected by CO₂. 19 of the mutants were stimulated by CO₂ and 2 were inhibited. The majority of the CO₂-stimulated mutants were confined to one section of the complementation map of the ad-2 locus.     

Identification of the structural gene for sucrase in Bacillus subtilis marburg

Mutants of Bacillus subtilis unable to grow on 0.1 p. cent sucrose were shown on the basis of enzymatic characterization and genetic mapping to be affected in either of two adjacent loci sacA and sacP. The sacP locus is defined by mutations impairing the activity of a phosphorylating sucrose transport system and the sacA locus by sucrase defective mutations. Proteins showing a crossreaction with antibodies directed against purified sucrase have been detected in crude extracts of two sacA mutants. According to these results it is proposed that sacA is the structural gene of sucrase and that the sacA and sacP loci are part of an operon.     

Mutants of Escherichia coli with temperature-sensitive lesions in membrane phospholipid synthesis: genetic analysis of glycerol-3-phosphate acyltransferase mutants

Summary Escherichia coli mutants possessing temperature-sensitive lesions in glycerol-3-phosphate acyltransferase, the enzyme catalysing the first step in phospholipid biosynthesis, have been characterized genetically. By recombinational and complementation tests, the mutants have been found to map in a single locus, called plsA, which is cotransducibile with the purE locus and lies between the purE and proC loci at minute 13 on the E. coli genetic map.     

Genetics and biochemistry of cycloheximide resistance in Physarum polycephalum

Summary Cycloheximide-resistant mutants of Physarum polycephalum were induced in the haploid myxamoebae by the combined action of UV¹ and caffeine (Haugli and Dove, 1972) or by treatment with NMG². Eight independent mutants segregated in a Mendelian fashion (Table 1). Crosses between 6 of the mutants revealed 2 loci, actA and actB, for cycloheximide resistance (Table 2).All mutants are expressed in the plasmodium and are recessive in heterozygotes (Fig. 1 and 2). One mutation, conferring resistance to high levels of cycloheximide, was studied in heterokaryons and found to be incompletely recessive.An in vitro peptide synthesizing system was constructed from ribosomes from Physarum and supernatant factors from Saccharomyces cerevisiae. Cycloheximide strongly inhibited the activity of ribosomes derived from either wild type or mutants at the actB locus. In contrast, ribosomes from mutants at the actA locus were resistant to cycloheximide. Thus, the actA locus operates through the ribosomes.     

Identification of two loci involved in phytochrome expression in Nicotiana plumbaginifolia and lethality of the corresponding double mutant

Four Nicotiana plumbaginifolia mutants exhibiting long hypocotyls and chlorotic cotyledons under white light, have been isolated from M2 seeds following mutagenesis with ethyl methane sulphonate. In each of these mutants, this partly etiolated in white light (pew) phenotype is due to a recessive nuclear mutation at a single locus. Complementation analysis indicates that three mutants, dap5, ems28 and ems3-6-34, belong to a single complementation group called pew1, while dap1 defines the pew2 locus. The mutants at pew1 contain normal levels of immunochemically detectable apoprotein of the phytochrome that is relatively abundant in etiolated seedlings, but are deficient in spectrophotometrically detectable phytochrome, whether seedlings are grown in darkness or light. Moreover, biliverdin, a precursor of the phytochrome chromophore, restores light-regulated responses in pew1 mutants and increases their level of photoreversible phytochrome when grown in darkness. These results indicate that the pew1 locus may be involved in chromophore biosynthesis. The mutant at the pew2 locus displays no photoreversible phytochrome in etiolated seedlings, but does contain normal levels of photoreversible phytochrome when grown in the light. Biliverdin had little effect on light-regulated responses in this mutant. In addition, biliverdin did not alter the level of phytochrome in etiolated seedlings. These observations lead us to propose that this mutant could be affected in the phyA gene itself. We have also obtained the homozygous double mutant at the pew1 and pew2 loci. This double mutant is lethal at an early stage of development, consistent with a critical role for phytochrome in early development of higher plants.     

mRNA distal to polar nonsense and insertion mutations in the gal operon of E. coli

Summary mRNA of the galactose operon of E. coli was measured in wildtype E. coli and in gal operon amber and insertion mutants. The mRNA coded by the distal half of the operon is reduced in the mutants. This reduction is more pronounced in the insertion mutants than in the amber mutants. It was compared with the polar effects of the mutations on the enzymes of the operon.     

A genetic assay for transversion mutations in bacteriophage T4

Summary Seventy-two mutants deficient in formate-nitrate reductase activity were selected in Escherichia coli strain PK 27, by two different procedures. Forty-five strains were selected on the basis of chlorate resistance and 27 strains were selected by their inability to reduce nitrate with formate as an electron donor. Genetic analysis of these strains showed that the two techniques yield distinctly different distributions of mutants among the various controlling genetic loci. Chlorate resistance appears to select for severe alterations in the nitrate reductase system; 98% of these mutants fell into the pleiotropic chl A, B, D and E classes and are deficient in all the activities of the formate-hydrogenlyase pathway as well as formate-nitrate reductase pathway. In contrast, 48% of the mutants selected for their inability to reduce nitrate with formate as the electron donor were of the chl C class and two new classes were identified among mutants selected by this procedure. Chl F mutants are linked to tryptophan and the chl C locus. Chl G mutants map at zero minutes on the E. coli genetic map.