Deoxyglucose-resistant mutants of Neurospora crassa: isolation, mapping, and biochemical characterization.

Neurospora crassa mutants resistant to 2-deoxyglucose have been isolated, and their mutations have been mapped to four genetic loci. The mutants have the following characteristics: (i) they are resistant to sorbose as well as to 2-deoxyglucose; (ii) they are partially or completely constitutive for glucose transport system II, glucamylase, and invertase, which are usually repressed during growth on glucose; and (iii) they synthesize an invertase with abnormal thermostability and immunological properties, suggesting altered posttranslational modification. All of these characteristics could arise from defects in the regulation of carbon metabolism. In addition, mutants with mutations at three of the loci lack glucose transport system I, which is normally synthesized constitutively by wild-type N. crassa. Although the basis for this change is not yet clear, the mutants provide a way of studying the high-affinity system II uncomplicated by the presence of the low-affinity system I.

Identification of Rhizobium phaseoli Strains That Are Tolerant or Sensitive to Soil Acidity.

A study was conducted to determine whether the survival of Rhizobium phaseoli in acid soils could be predicted on the basis of the tolerance of the organism to acidity in culture. Of 16 strains tested, all grew in culture at pH 4.6, but only those that grew at pH 3.8 survived in soils having pH values of 4.1 to 4.6. Strains that tolerated the lowest pH values in culture were tolerant of the highest aluminum concentrations. In one acid soil, an acid-tolerant strain was unable to survive in numbers greater than 100/g, but the poor survival was not related to the level of extractable aluminum or manganese in the soil. Reproduction of an acid-tolerant strain of R. phaseoli was enhanced in the rhizosphere of Phaseolus vulgaris in both acid and limed soils, but stimulation of an acid-sensitive strain by the plant occurred only in the limed soil. These results indicate that cultural tests can be used to predict the ability of R. phaseoli to survive in acid soil.

Survival of Rhizobium in Acid soils.

A Rhizobium strain nodulating cowpeas did not decline in abundance after it was added to sterile soils at pH 6.9 and 4.4, and the numbers fell slowly in nonsterile soils at pH 5.5 and 4.1. A strain of R. phaseoli grew when added to sterile soils at pH 6.7 and 6.9; it maintained large, stable populations in soils of pH 4.4, 5.5, and 6.0, but the numbers fell markedly and then reached a stable population size in sterile soils at pH 4.3 and 4.4. The abundance of R. phaseoli added to nonsterile soils with pH values of 4.3 to 6.7 decreased similarly with time regardless of soil acidity, and the final numbers were less than in the comparable sterile soils. The minimum pH values for the growth of strains of R. meliloti in liquid media ranged from 5.3 to 5.9. Two R. meliloti strains, which differed in acid tolerance for growth in culture, did not differ in numbers or decline when added to sterile soils at pH 4.8, 5.2, and 6.3. The population size of these two strains was reduced after they were introduced into nonsterile soils at pH 4.8, 5.4, and 6.4, and the number of survivors was related to the soil pH. The R. meliloti strain that was more acid sensitive in culture declined more readily in sterile soil at pH 4.6 than did the less sensitive strain, and only the former strain was eliminated from nonsterile soil at pH 4.8; however, the less sensitive strain also survived better in limed soil. The cell density of the two R. meliloti strains was increased in pH 6.4 soil in the presence of growing alfalfa. The decline and elimination of the tolerant, but not the sensitive, strain was delayed in soil at pH 4.6 by roots of growing alfalfa.

Genetic regulation of phosphate transport system II in Neurospora.

Phosphate transport system II, previously shown to be responsible for high-affinity phosphate uptake under conditions of phosphorus starvation, is regulated by at least three genes: pcon-nuc-2, preg, and nuc-1. nuc-1 and nuc-2 mutants cannot be derepressed for phosphate transport system II, while pconc and pregc mutants are partially constitutive.

Phosphate transport in Neurospora. Derepression of a high-affinity transport system during phosphorus starvation.

In addition to the constitutive, low-affinity phosphate-transport system described previously, Neurospora possesses a second, high-affinity system which is derepressed during phosphorus starvation. At pH 5.8, System ii has a K1/2 of about 3muM and a Jmax of 5.2 mmol/1 cell water per min. System ii reaches maximal activity after about 2 h of growth in phosphorus-free minimal medium. Its formation is blocked by cycloheximide and, once made, it appears to turn over rapidly. Addition of cycloheximide to fully derepressed cultures results in the decay of System ii with a t1/2 of 14 min, very similar to the turnoacteriol. 95, 959-966) for tryptophan transport in Neurospora. Thus, these transport systems appear to be regulated by a balance between synthesis and breakdown, as affected by intracellular pools of substrate or related compounds.

Sodium dynamics in a northern ecosystem.

Analyses of terrestrial sources of sodium and estimates of the sodium requirement of moose (Alces alces) on Isle Royale, Lake Superior, suggest that availability of the element controls the moose population. The terrestrial vegetation is very poor in the element, but, as elsewhere, submerged and floating leaved water-plants are relatively rich. Consumption of such plants in summer would provide an adequate source, if the animal can store the element. The fairly high sodium contents of freshwater vegetation have been little appreciated. In general, sodium concentration in water-plants, unlike that of potassium, is not correlated with chloride but the latter is ordinarily in excess of the sodium, so that uptake of the latter implies an equivalent supply of NaCl.