Effect of mutations in the qa gene cluster of Neurospora crassa on the enzyme catabolic dehydroquinase

Gene organization and regulation in the qa (quinic acid) gene cluster of Neurospora crassa

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Proof of de novo synthesis of the qa enzymes of Neurospora crassa during induction

In Neurospora crassa three inducible enzymes are necessary to catabolize quinic acid to protocatechuic acid. The three genes encoding these enzymes are tightly linked on chromosome VII near methionine-7 (me-7). This qa cluster includes a fourth gene, qa-1, which encodes a regulatory protein apparently exerting positive control over transcription of the other three qa genes. However, an alternative hypothesis is that the qa-1 protein simply activates preformed polypeptides derived from the three structural genes. The use of density labeling with D(2)O demonstrated conclusively that the qa enzymes are synthesized de novo only during induction on quinic acid. Native catabolic dehydroquinase (5-dehydroquinate dehydratase; 5-dehydroquinate hydro-lyase, EC 4.2.1.10) (a homopolymer of ca 22 identical subunits) has a density of 1.2790 g/cm(3) as determined by centrifugation in a modified cesium chloride density gradient. Growth in H(2)O followed by induction in 95% D(2)O shifts the density of the enzyme to 1.3130 g/cm(3), indicating de novo synthesis during induction. In the reciprocal experiment, i.e., growth in 80% D(2)O followed by induction in either 95% D(2)O or H(2)O, the densities of catabolic dehydroquinase were 1.3135 and 1.2800 g/cm(3), respectively. Because growth on D(2)O does not affect the density of the H(2)O-induced enzyme, there can be no significant synthesis of catabolic dehydroquinase prior to induction. Similar results were obtained for a second qa enzyme, quinate dehydrogenase (quinate:NAD(+) oxidoreductase, EC 1.1.1.24). Thus, induction of two qa enzymes involves de novo protein synthesis, not enzyme activation or assembly.

Expression in Escherichia coli K-12 of the structural gene for catabolic dehydroquinase of Neurospora crassa

The inducible quinic acid catabolic pathway of Neurospora crassa is controlled by four genes, the qa cluster which includes structural genes qa-2, qa-3, qa-4 for three enzymes and a regulatory gene, qa-1. In this paper we report the molecular cloning of at least the qa-2 gene which encodes the catabolic dehydroquinase (5-dehydroquinate hydro-lyase, EC 4.2.1.10). Endo.R.HindIII restriction endonuclease fragments of N. crassa DNA from a qa-1(c) (constitutive) mutant and of Escherichia coli plasmid pBR322 DNA were ligated in vitro and used to transform an aroD6 (5-dehydroquinate hydrolyase deficient) strain of E. coli K12. The recombinant plasmid (pVK55) isolated from one AroD(+) transformant (SK1518) contained, in addition to pBR322, two N. crassa HindIII fragments with molecular weights of 2.3 x 10(6) and 1.9 x 10(6). Derivatives of SK1518 cured of plasmid DNA were phenotypically Amp(s) and AroD(-). These cured strains, retransformed with pVK55, were phenotypically Amp(R) and AroD(+). Strains transformed with pVK55 possessed 5-dehydroquinate hydrolyase activity but no activity was present in any AroD(-) strain. The enzyme extracted from strains containing the recombinant plasmid was identical to N. crassa catabolic dehydroquinase by the criteria of heat stability, ammonium sulfate fractionation, immunological crossreactivity, molecular weight, and purification characteristics. This identity demonstrates that the N. crassa qa-2(+) gene is carried by the recombinant plasmid and is apparently transcribed and translated with complete fidelity. Furthermore, subunit assembly of the N. crassa polypeptides also occurs in E. coli, because the catabolic dehydroquinase is a multimer composed of approximately 20 identical subunits.

Characterization and in vitro translation of polyadenylated messenger ribonucleic acid from Neurospora crassa

Ribonucleic acid (RNA) extracted from Neurospora crassa has been fractionated by oligodeoxythymidylic acid [oligo(dT)]-cellulose chromatography into polyadenylated messenger RNA [poly(A) mRNA] and unbound RNA. The poly(A) mRNA, which comprises approximately 1.7% of the total cellular RNA, was further characterized by Sepharose 4B chromatography and polyacrylamide gel electrophoresis. Both techniques showed that the poly(A) mRNA was heterodisperse in size, with an average molecular weight similar to that of 17S ribosomal RNA (rRNA). The poly(A) segments isolated from the poly(A) mRNA were relatively short, with three major size classes of 30, 55, and 70 nucleotides. Gel electrophoresis of the non-poly(A) RNA indicated that it contained primarily rRNA and 4S RNA. The optimal conditions were determined for the translation of Neurospora mRNA in a cell-free wheat germ protein-synthesizing system. Poly(A) mRNA stimulated the incorporation of [14C]leucine into polypeptides ranging in size from 10,000 to 100,000 daltons. The RNA that did not bind to oligo(dT)-cellulose also stimulated the incorporation of [14C]leucine, indicating that this fraction contains a significant concentration of mRNA which has either no poly(A) or very short poly(A) segments. In addition, the translation of both poly(A) mRNA and unbound mRNA was inhibited by 7-methylguanosine-5'-monophosphate (m7G5'p). This is preliminary evidence for the existence of a 5'-RNA "cap" on Neurospora mRNA.

Characterization of qa-2 mutants of Neurospora crassa by genetic, enzymatic, and immunological techniques

Genetic and complementation mapping studies using 20 qa-2 mutants defective for catabolic dehydroquinase indicate that the qa-2 gene encodes a single polypeptide chain and is the structural gene for catabolic dehydroquinase, a 220,000-molecular-weight protein composed of identical 10,000-molecular-weight subunits. Many qa-2 mutants are capable of reversion, but no evidence has yet been obtained for nonsense mutations in this gene. The biochemical consequences of the mutations in two complementing qa-2 strains (M239 and M204) have been determined. Both mutants have extremely low levels of catalytic activity and form a heterocaryon with about 4% of the wild-type activity. As assayed by immunological cross-reactivity, mutant M239 and the heterocaryon have nearly wild-type levels of native-molecular-weight catabolic dehydroquinase protein, whereas M204 has no detectable amount of this protein. Thus it is concluded that M239 has a mutation at or near the catalytic site which reduces the activity 10,000-fold but has little or no influence on the formation of the native multimeric structure. In contrast, M204 apparently has a mutation that severely inhibits aggregation and may have only a minor effect on the inherent potential for catalytic conversion at the reactive site. The heterocaryon would appear to form a mixed multimer with the monomeric subunits from M239 providing the aggregated structure and those from M204, the catalytically active moiety.