Induction of ?2 frameshift mutations within alternating GC sequences by carcinogens that bind to the C8 position of guanine residues Development of a specific mutation assay

Summary Using a forward mutation assay we have previously found that N-2-acetylaminofluorene (AAF), a strong chemical carcinogen, induces a majority of frameshift mutations located at specific sequences called mutation hot spots. Among these hot spot sequences, the NarI sequence (GGCGCC), is specific for –2 frameshifts (GGCGCC) GGCC). Interestingly, these frameshift mutations occur independently of a functional umuDC locus. Being interested in elucidating this mutation pathway we have developed a reversion assay that is specific for this class of mutations. The assay is based on the reversion of a +2 frameshift mutant of plasmid pBR322 from tetracycline sensitivity to tetracycline resistance. It is shown that only true reversion events lead to tetracycline resistance. The carcinogen AAF induces this reversion event at a frequency that is increased four- to fivefold over the background frequency. A series of chemical carcinogens which, like AAF, bind covalently to the C8 position of guanine, are compared for their efficiency to induce this specific mutation event. Large variations in the mutagenic efficiency of these chemicals are observed and discussed in terms of the anti/syn conformation of the carcinogen-modified guanine residue. Based on this test, we describe a convenient spot assay that this presently used in our laboratory to isolate Escherichia coli mutants affected in this mutation pathway.     

Detection of a protein encoded by a class II mitochondrial intron of Podospora anserina

Summary In the filamentous fungus Podospora anserina, the amplification as circular DNA molecules of the first intron (intron ) of the CO1 mitochondrial gene, encoding the cytochrome oxidase subunit 1, is known to be strongly associated with aging of strains. In this study we have attempted to detect the protein potentially encoded by the open reading frame (ORF) contained in this intron. This was done by the Western blot technique using specific antisera raised against three polypeptides encoded by three non-overlapping fragments of this ORF adapted to the universal code and overexpressed in Escherichia coli. We examined about thirty independent subclones of Podospora derived from two different geographic races (A, s), using wild-type and mutant strains, young and senescent cultures. A 100 kDa polypeptide, encoded by the class II intron , was detected in five senescent subclones which all showed strong amplification of the intronic sequence (Sen DNA ).     

Mutagenicity, carcinogenicity and teratogenicity of cobalt metal and cobalt compounds

Cobalt metal and cobalt compounds are extensively used for the production of high-temperature alloys, diamond tools, cemented carbides and hard metals, for the production of various salts used in electroplating and as catalysts, drying agents in paints, additives in animal feeds and pigments. Cobalt oxides are used not only in the enameling industry and for pigments, but also in catalytic applications. There is no indication that cobalt metal and cobalt compounds constitute a health risk for the general population. Allergic reactions (asthma, contact dermatitis) can be induced by certain cobalt compounds. Interstitial fibrosis has also been observed in workers exposed to high concentrations of dust containing cobalt, tungsten, iron, etc., mainly in the cemented carbides and the diamond-polishing industries. Several experiments have demonstrated that single or repeated injections of cobalt metal powder or some forms of cobalt salt and cobalt oxide may give rise to injection site sarcoma in rats and in rabbits but the human health significance of such data is questionable. Intratracheal administration of a high dose of one type of cobalt oxide induces lung tumors in rats but not in hamsters. In the latter long-term inhalation of cobalt oxide (10 mg/m3) did not increase the incidence of lung cancer. The human data are too limited to assess the potential carcinogenic risk for workers. Co2+ interacts with protein and nucleic acid synthesis and displays only weak mutagenic activity in microorganisms. Some cobalt salts have been reported to enhance morphological transformation of Syrian hamster embryo cells. Cobalt chloride displays some limited mutagenic activity in yeast and some cobalt compounds are able to produce numerical and structural chromosome aberrations in plant cells. Cobalt and its salts appear to be devoid of mutagenic and clastogenic activity in mammalian cells. Cobaltous acetate and cobaltous chloride have not been found to be teratogenic in hamsters and rats respectively.     

Sexual and site differences in calcium consumption by the Malabar Giant Squirrel Ratufa indica

Summary The calcium ingestion of Malabar Giant Squirrels Ratufa indica (Sciuridae) was examined at two sites, Magod and Bhimashankar, in western India. In females at Magod a positive correlation was found between rates of calcium ingestion from food resources and the contribution of those resources to the daily diet. This relationship was true for all females at Magod irre-spective of their reproductive condition. This relationship was neither significant for males at the same site nor for both sexes at Bhimashankar. A differential requirement for calcium between the sexes and the occurrence of higher rates of calcium ingestion or the probable presence of more easily digestible calcium at Bhimashankar are postulated to explain the observed phenomenon. Mature leaves and bark appear to be reliable sources of calcium at these sites.     

Correlation of xylem sap cytokinin levels with monocarpic senescence in soybean

Cytokinins (CKs) coming from the roots via the xylem are known to delay leaf senescence, and their decline may be important in the senescence of soybean (Glycine max) plants during pod development (monocarpic senescence). Therefore, using radioimmunoassay of highly purified CKs, we quantified the zeatin (Z), zeatin riboside (ZR), the dihydro derivatives (DZ, DZR), the O-glucosides, and DZ nucleotide in xylem sap collected from root stocks under pressure at various stages of pod development. Z, ZR, DZ, and DZR dropped sharply during early pod development to levels below those expected to retard senescence. Pod removal at full extension, which delayed leaf senescence, caused an increase in xylem sap CKs (particularly ZR and DZR), while depodding at late podfill, which did not delay senescence, likewise did not increase the CK levels greatly. The levels of the O-glucosides and the DZ nucleotide were relatively low, and they showed less change with senescence or depodding. The differences in the responses of individual CKs to senescence and depodding suggest differences in their metabolism. Judging from their activity, concentrations and response to depodding, DZR and ZR may be the most important senescence retardants in soybean xylem sap. These data also suggest that the pods can depress CK production by the roots at an early stage and this decrease in CK production is required for monocarpic senescence in soybean.     

A Direct Screening Procedure for Gravitropism Mutants in Arabidopsis thaliana (L.) Heynh.

In order to isolate gravitropism mutants of Arabidopsis thaliana (L.) Heynh. var Estland for the genetic dissection of the gravitropism pathway, a direct screening procedure has been developed in which mutants are selected on the basis of their gravitropic response. Variability in hypocotyl curvature was dependent on the germination time of each seed stock, resulting in the incorrect identification of several lines as gravitropism mutants when a standard protocol for the potentiation of germination was used. When the protocol was adjusted to allow for differences in germination time, these lines were eliminated from the collection. Out of the 60,000 M2 seedlings screened, 0.3 to 0.4% exhibited altered gravitropism. In approximately 40% of these mutant lines, only gravitropism by the root or the hypocotyl was altered, while the response of the other organ was unaffected. These data support the hypothesis that root and hypocotyl gravitropism are genetically separable.     

Rhizobium meliloti exopolysaccharide Mutants Elicit Feedback Regulation of Nodule Formation in Alfalfa

Nodule formation by wild-type Rhizobium meliloti is strongly suppressed in younger parts of alfalfa (Medicago sativum L.) root systems as a feedback response to development of the first nodules (G Caetano-Anollés, WD Bauer [1988] Planta 175: 546-557). Mutants of R. meliloti deficient in exopolysaccharide synthesis can induce the formation of organized nodular structures (pseudonodules) on alfalfa roots but are defective in their ability to invade and multiply within host tissues. The formation of empty pseudonodules by exo mutants was found to elicit a feedback suppression of nodule formation similar to that elicited by the wild-type bacteria. Inoculation of an exo mutant onto one side of a split-root system 24 hours before inoculation of the second side with wild-type cells suppressed wild-type nodule formation on the second side in proportion to the extent of pseudonodule formation by the exo mutants. The formation of pseudonodules is thus sufficient to elicit systemic feedback control of nodulation in the host root system: infection thread development and internal proliferation of the bacteria are not required for elicitation of feedback. Pseudonodule formation by the exo mutants was found to be strongly suppressed in split-root systems by prior inoculation on the opposite side with the wild type. Thus, feedback control elicited by the wild-type inhibits Rhizobium-induced redifferentiation of host root cells.     

Simulated and measured nitrogen conditions in a manured and fertilised soil

The governing factors for soil nitrogen dynamics were identified with a simulation model. In addition, the model was used to interpret measurements from a plot fertilisation experiment in southwest Sweden.Simulated moisture and temperature conditions were the driving variables for the simulation of soil nitrogen dynamics and leaching during a 6-year period. The results of the simulation were compared with monthly observations on two plots with grain crops, one with liquid manure and commercial fertilisers applied and one with commercial fertilisers only.Simulated temporal variations of the nitrate and ammonium storages generally agreed with observations. The dominant role of the crops as a determinant of soil nitrogen conditions was demonstrated. A higher leaching loss from the plot with application of commerical fertilisers only occurred both in simulations and measurements compared to the plot with application of both commercial fertilisers and manure. The main reason was the higher N-application in the former treatment.The effect of water flows in macropores was interpreted as a delay of simulated leaching compared to observed leaching on some occasions in summer and early autumn. No direct effect of the macropores on the yearly rates of leaching could be seen.