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Neurospora crassa Cytoplasmic Ribosomes: Isolation and Characterization of a Cold-Sensitive Mutant Defective in Ribosome Biosynthesis 总被引:11,自引:4,他引:7
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Twenty-seven cold-sensitive mutants of Neurospora crassa were isolated by mutagenesis of wild-type conidia followed by filtration enrichment in complete medium at the nonpermissive temperature (10 C). Zone sedimentation analyses of cytoplasmic ribosomes isolated from the wild-type strain and from 14 of the mutant strains grown at 10 C indicate that one cold-sensitive mutant is defective in ribosome biosynthesis at that temperature: instead of the 2.3:1 mass ratio of 60S:37S ribosomal subunits characteristic of wild type, the mutant strain PJ30201 (called crib-1 for cytoplasmic ribosome biosynthesis) exhibits a mass ratio of approximately 7.2:1. Ribosomal subunits synthesized by strain PJ30201 at 25 C are present in wild-type proportions. The cold-sensitive and ribosomal phenotypes segregate together in tetrads isolated from crosses between strain PJ30201 and the wild type indicating that a single nuclear gene mutation is probably responsible for both mutant phenotypes. The crib-1 locus lies near the centromere in linkage group IV. 相似文献
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The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
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