Antisuppressor mutations in Aspergillus nidulans: cold-resistant revertants of suppressor suaC109

Isolation and nucleotide sequence of the ribosomal protein S16-encoding gene from Aspergillus nidulans

A genomic clone has been isolated from Aspergillus nidulans which is homologous to the ribosomal (r) protein S16-encoding gene of Saccharomyces cerevisiae (S16A) and the r-protein S19-encoding gene of rat (S19). The amino acid (aa) sequences, deduced from nucleotide (nt) sequence analysis, show that in both cases more than 63% of the aa are conserved. The proposed A. nidulans r-protein S16 gene (rps16) differs from that of S. cerevisiae in that it occurs as a single copy in the haploid genome (rather than two copies as in yeast) and contains two putative introns (rather than one). The mRNA leader is long compared to many Aspergillus genes, commencing 293 nt upstream from the coding region, and contains an open reading frame of 13 codons.

Hygromycin- and paromomycin-resistant mutants of Aspergillus nidulans alter translational fidelity

Mutants of Aspergillus nidulans resistant to the aminoglycoside antibiotics paromomycin and hygromycin B have been isolated and their growth characteristics are described here. Most paromomycin mutants were cross-resistant to hygromycin and geneticin. All the hygromycin-resistant mutants were slightly cross-resistant to geneticin. Out of the 15 mutants tested 14 had drug-resistant ribosomes in vitro and all 12 of those investigated further had reduced levels of translational misreading. Five new loci have been found--parA on linkage group I, hygA on III, hygB on IV, hygC on V, hygD on VI and parB on VIII. This increases, to at least 12, the number of translational fidelity loci in A. nidulans.

Antisuppressor mutations reduce misreading of the genetic code in Aspergillus nidulans

Antisuppressor mutations were isolated in a strain containing the omnipotent suppressor suaC109. The antisuppressors reduce the activity of translational suppressors in vivo and counteract most aspects of the pleiotropic phenotype associated with the suaC and the suaA suppressor mutations. Using an homologous system for cell-free translation, we have measured translational accuracy in two antisuppressor strains with the genotype suaC109 and either the asuB11 or the asuD14 antisuppressor mutation. Ribosomes from antisuppressor mutants have higher levels of translation accuracy than those from the suppressor strain (suaC109, asu+). Mistranslation levels depended solely on the source of the sucrose-cleaned ribosomes. However, the increased accuracy associated with sucrose-cleaned ribosomes from antisuppressor strains can be nullified by salt-washing, suggesting that the component responsible can be washed off.

Ammonium ion sensitivity is a ribosomal phenotype associated with suppressor mutations in the suaC gene of Aspergillus nidulans

Ammonium ions are selectively toxic to strains containing mutations in the suaC gene which can mutate to a suppressor phenotype. This phenotype is associated with increased ribosomal misreading in vitro (Zamir and Martinelli 1987) and altered ribosomal proteins (Harvey and Martinelli 1983). Such ammonium-sensitivity is a feature of both strong and weak suppressor alleles, and segregates with suppressor ability in crosses. Suppressor mutations in the suaB and suaD genes are not affected, nor are those in suaA, another ribosomal suppressor gene. Thus, the ammonium-effect is locus specific. Mutations which act as anti-suppressors (asu-) of suppressor suaC109 also partially reverse the ammonium ion sensitivity associated with this mutation. This effect is in line with their restoration of other aspects of the pleiotropic phenotype to normal. The cations, lithium and rubidium, mimick the effects of ammonium ions. Only ribosomes from suaC strains are sensitive to the presence of NH+4 ions in vitro.

Every ribosomal suppressor mutation in Aspergillus nidulans has a unique and highly pleiotropic phenotype

18 suppressors of alcR125 have been selected in Aspergillus nidulans. They have been located in genes as follows: 12 in suaA, 1 in suaB and 5 in suaC. Suppressors have been examined to see whether their phenotype is diagnostic for their genotype. Several new traits are described: conidial viability, cycloheximide resistance, fertility, suppression of niaD500, niaD501 and fwA1. These tests, added to those already in use, provide a battery of tests suitable for assigning suppressor mutations to physiological type (tRNA or ribosomal), and in one case to a specific gene since only suaA mutations suppressed fwA1. A very broad range of phenotypes was associated with suppressors such that every mutation had a unique phenotype. This indicates that the ribosomal suppressor mutations are in genes which code directly for ribosomal proteins, rather than genes which code for modifying enzymes.