Biochemical Aspects of Genetics

It would be impossible, in the time and space available, to cover all of the noteworthy advances that have been made in biochemical genetics during the past year. We have, therefore, selected for review a number of topics that are especially active at the moment and that promise to yield important new results in the near future. At the same time, we have tried to avoid duplicating the material of other chapters in this volume which are germane to biochemical genetics. We refer, in particular, to the chapters on Metabolism of Nucleic Acids (Macromolecular DNA and RNA); Nucleic Acids and Protein Biosynthesis; The Basic Proteins of Cell Nuclei; and Chemistry of Differentiation in Lower Organisms. The reader should consult these reviews, as well as the present one, for a fuller view of current activities in this field

Vogel\u27s Medium N salts: avoiding the need for ammonium nitrate

Vogel\u27s Medium N salts (Vogel, 1956 Microbiol. Genet. Bull. 13:42-43), supplemented with a carbon source, is widely used for the cultivation of Neurospora and many other fungi. The standard recipe includes ammonium nitrate

Construction of minimally-sheltered knockouts mutants of Neurospora crassa

I have been developing strains which I think will make it possible to produce minimally-sheltered knockouts of essential genes routinely. They could equally well be called self-adjusting knockdowns of essential genes. Such minimally-sheltered knockouts could give results in microarray analysis that would be less subject to artifact than results with heterokaryons or with homokaryons sheltered with an inducible wild-type allele of the gene in question. The method addresses two questions about each cloned gene, henceforth and collectively called your favorite gene (yfg). yfg should be one which does not pass successfully through a cross as a hygromycin-resistant knockout. (1) Is yfg a dispensable gene? (2) If yfg is found to be essential, what is the terminal phenotype of the yfgΔ knockout mutation? By terminal phenotype, I mean that of the heterokaryon between yfgΔ and a partial diploid, yfg+/Δ, in which the partial diploid is the minimum proportion that allows somewhat suboptimal growth

Bird Medium: an alternative to Vogel Medium

This medium was designed to circumvent some problems that arise in the use of Medium N (Vogel 1964 Am. Naturalist 98:435-446). These are, among others, the presence of high levels of citrate, a chelator which leaves the concentration of calcium and trace elements uncertain; the use of ammonium nitrate, which leaves the actual source of nitrogen ambiguous; the use of MgSO4, which does not allow the experimenter to vary the concentration of magnesium and sulfur independently; the high activity coefficient for the pKa values of citrate, which makes the pH unnecessarily sensitive to ionic strength; the use of sucrose, which leaves uncertain the nature and relative amounts of the hexose(s) being used at any particular moment; the need to use chloroform as a preservative, which results in the gradual depletion of the aqueous phase of complexes of trace elements

Improved selection for inositol-utilization following transformation of Neurospora crassa

Plasmids based on the cloned inositol gene of Neurospora crassa are potentially very useful as a transformation marker, or for techniques like insertional mutagenesis

A simple way to make a dilution series

Often one needs to determine a suitable concentration of a previously untested nutrient or inhibitor to use in subsequent experiments

A minichromosome of LGVI from crossing two quasi-terminal reciprocal translocations

Non-reciprocal translocations, in which one chromosome is a pure donor and another is a pure recipient, have found abundant uses in genetics, molecular biology, and cytology (Perkins 1997Advances in Genetics 36:239-398). Our original aim was to prepare a strain carrying a chromosome truncated in both arms, with the idea that such a small chromosome would be easily purified by pulsed field electrophoresis and would be a good preliminary substrate for genomic sequencing

Meiotic Silencing by Unpaired DNA

AbstractThe silencing of gene expression by segments of DNA present in excess of the normal number is called cosuppression in plants and quelling in fungi. We describe a related process, meiotic silencing by unpaired DNA (MSUD). DNA unpaired in meiosis causes silencing of all DNA homologous to it, including genes that are themselves paired. A semidominant Neurospora mutant, Sad-1, fails to perform MSUD. Sad-1 suppresses the sexual phenotypes of many ascus-dominant mutants. MSUD may provide insights into the function of genes necessary for meiosis, including genes for which ablation in vegetative life would be lethal. It may also contribute to reproductive isolation of species within the genus Neurospora. The wild-type allele, sad-1+, encodes a putative RNA-directed RNA polymerase

Reversal of a Neurospora Translocation by Crossing over Involving Displaced Rdna, and Methylation of the Rdna Segments That Result from Recombination

In translocation OY321 of Neurospora crassa, the nucleolus organizer is divided into two segments, a proximal portion located interstitially in one interchange chromosome, and a distal portion now located terminally on another chromosome, linkage group I. In crosses of Translocation x Translocation, exceptional progeny are recovered nonselectively in which the chromosome sequence has apparently reverted to Normal. Genetic, cytological, and molecular evidence indicates that reversion is the result of meiotic crossing over between homologous displaced rDNA repeats. Marker linkages are wild type in these exceptional progeny. They differ from wild type, however, in retaining an interstitial block of rRNA genes which can be demonstrated cytologically by the presence of a second, small interstitial nucleolus and genetically by linkage of an rDNA restriction site polymorphism to the mating-type locus in linkage group I. The interstitial rDNA is more highly methylated than the terminal rDNA. The mechanism by which methylation enzymes distinguish between interstitial rDNA and terminal rDNA is unknown. Some hypotheses are considered