Complementation between alleles of the Td locus in Neurospora crassa

Molecular analysis of intragenic recombination at the tryptophan synthetase locus in Neurospora crassa

Fifteen different classically generated and mapped mutations at the tryptophan synthetase locus in Neurospora crassa have been characterized to the level of the primary sequence of the gene. This sequence analysis has demonstrated that intragenic recombination is accurate to order mutations within one open reading frame. While classic genetic analysis correctly ordered the mutations, the position of mutations characterized by gene sequence analysis was more accurate. A leaky mutation was found to have a wild-type primary sequence. The presence of unique polymorphisms in the primary sequence of the trp-3 gene from strain 861 confirms that it has a unique history relative to the other strains studied. Most strains that were previously shown to be immunologically nonreactive with antibody preparations raised against tryptophan synthetase protein were shown to have nonsense mutations. This work defines 14 alleles of the N. crassa trp-3 gene.

Orientation of enzymic domains in tryptophan synthase of Neurospora crassa: an immunoblot analysis of TRP3 mutant products

Extracts of 52 TRP3 mutants of Neurospora crassa were tested for the presence of serologically cross-reacting material by the method of electrophoretic blot analysis. The test antigen was obtained by excision of lightly stained bands of denatured pure tryptophan synthase after SDS-polyacrylamide gel electrophoresis. Rabbit antisera raised against this antigen neutralized and precipitated native tryptophan synthase. Of the 52 strains, 19 exhibited banding patterns similar to wild type on electrophoretic blots, 25 strains gave no apparent bands, and 8 strains showed unique banding patterns. This evidence and related genetic fine structure mapping data indicate that strains exhibiting banding patterns similar to wild type carry missense mutations. Strains which did not exhibit any obvious bands may have resulted from certain kinds of missense or nonsense mutations or from frameshift mutations or extended deletions. Strains exhibiting unique banding patterns on electrophoretic blots were interpreted as carrying chain-terminating mutations or deletions. Genetic fine structure mapping data place the mutant lesions of these strains in a linear order corresponding to the apparent molecular weights of the crossreacting protein fragments which they exhibit. The direction of transcription and translation of the TRP3 locus in Neurospora was inferred from these relationships. The apparent organization of the Neurospora TRP3 gene is consistent with that ascribed to the Saccharomyces TRP5 system and suggests that the N-terminal portion of the protein corresponds to the "A" protein of the Escherichia coli system and that the C-terminal portion of the protein corresponds to the "B" protein of the bacterial system.

Two mechanisms of allelic complementation among tryptophan synthetase mutants of Saccharomyces cerevisiae

Two different types of allelic complementation were observed in tryptophan synthetase mutants of the yeast Saccharomyces cerevisiae. Each type is associated with a different mechanism for the enzymatic conversion of indole-3-glycerol phosphate (InGP) to tryptophan. Mechanism I is utilized by a hybrid tryptophan synthetase that resembles, but is not identical with, the wild-type enzyme. Mechanism II is due to a sequential conversion of InGP to free indole, and indole to tryptophan. Two partially active mutant enzymes rather than a single hybrid enzyme catalyze the sequential reaction steps. This is an example of intracellular cross-feeding. The quantitative evaluation of mechanism II leads to the conclusion that tryptophan synthetase in yeast is most likely a dimer of two identical subunits.

A ribonucleic acid fraction of Neurospora crassa evoking development of a pseudo-tryptophan synthetase activity by extracts of mutants TD-1, TD-2, and TD-3

"Soluble RNA" fractions isolated from the wild-type strain of Neurospora crassa evoked development of a pseudo-tryptophan synthetase enzyme activity in vitro by extracts of mutant strains lacking ability to produce the enzymic activity. The RNA fractions contained no detectable "template RNA". Some properties of the component eliciting development of pseudo-tryptophan synthetase activity are reported.