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J A Roper

Publications and source records attributed to J A Roper.

At least 19 recordsLinked to original sources
PubMed1985

Spontaneous IR duplications generated at mitosis in Aspergillus nidulans: further evidence of a preferential site of transposed attachment.

A radiation-induced translocation, T(IIR----IIIL), has been shown to be nonreciprocal and to have most of IIR, including its terminus, attached uninverted to the terminus of IIIL.--Progeny with the IIR segment in duplicate, obtained from crosses of T(IIR----IIIL) to strains with a standard genome, were unstable at mitosis; like earlier duplication strains, they suffered deletions from either duplicate segment. Frequent mitotic crossing over occurred between the duplicate IIR segments so that, following deletions, more than two classes of stable, balanced products arose from each heterozygous duplication strain.-- Spontaneous, mitotically arising duplications of the IR segment, bearing the rate-limiting adE20 allele, can be selected on adenine-free medium on which they emerge as vigorous sectors from the stunted adE20 colony. It was shown previously that most such duplications, when selected from a strain with standard genome, had the terminal IR segment attached to the end of IIR. Selection has now been made from an adE20 strain carrying T(IIR----IIIL), and seven of the 13 independent IR duplications were linked to the III-IIR translocation complex. In three strains analyzed further, the duplicate IR segments, which included the IR terminus, were attached uninverted to the terminus of IIR; the segments of IR were of approximately equal genetic length.--This supports earlier suggestions that there is a preferential site for the initiation of IR duplications and a preferential site, the IIR terminus, for their attachment.

Aspergillus nidulans↗
PubMed1984

Spontaneous duplications and transpositions of a large chromosome segment in Aspergillus nidulans.

Spontaneous revertants of the leaky adE20 mutant of Aspergillus nidulans were obtained as vigorous sectors emerging from stunted colonies on adenine-free medium. Among the genetically heterogeneous sectors up to about 20% were recognized unequivocally as having an additional chromosome segment bearing adE20; two doses of this leaky allele permitted growth without added adenine. Eleven spontaneous duplication strains of independent origin were analysed genetically. Eight carried the duplicate segment on chromosome IIR; three of these, phenotypically similar to all eight, were analysed in detail and were shown-within the limits of such genetic analysis - to have a large, terminal segment of IR duplicated and attached terminally and uninverted to IIR. One strain had a duplication, possibly tandem, on IR and two had duplications attached elsewhere in the genome. The results suggested a preferential site for the initiation of duplicate segments in this system, as well as a preferential site for their attachment. Agents known to modify instability of a previously studied Dp(IR----IIR) strain affected the frequency of duplications among selected adE20 revertant sectors and/or the genomic locations of duplicated segments. Trypan blue and coumarin, which enhance Dp(IR----IIR) instability in a specific way, and Co2+, which stabilizes Dp(IR----IIR), gave 14, 50 and 62% duplication sectors respectively, among revertants. Duplications selected in the presence of Co2+ had mainly IIR attachments; of those from trypan blue and coumarin, about one-quarter were attached to IIL and none to IIR.

Aspergillus nidulans↗
PubMed1982

Modification of chromosome instability in Aspergillus nidulans.

Strains of Aspergillus nidulans with a chromosome segment in duplicate show instability at mitosis; their colonies produce faster-growing sectors which arise from nuclei with spontaneous deletions in either duplicate segment. In an attempt to probe the deletion process, the effects of mutations causing sensitivity to UV treatment, and those of manganous ions, have been studied in strains carrying either Dp(I,II) or Dp(III,VIII). For comparison, the effects of Mn2+ on balanced and unbalanced diploids have also been examined. The uvsE allele, which decreases intragenic mitotic crossing over in diploids, increased deletion frequency in strains with either duplication. The uvsB allele, which increases intragenic mitotic crossing over in diploids, increased deletion frequency only in Dp(I,II) strains; in addition, by causing early mitotic crossing over between the homologous segments, it produced some novel deletion products. Mn2+ substantially decreased the deletion frequency in Dp(I,II) strains and decreased mitotic crossing over in diploids; it had no effect on Dp(III,VIII) strains. The results suggest that in haploid duplication strains there are two classes of spontaneous DNA lesions, recombinogenic and non-recombinogenic, both of which, failing repair, lead to deletion.

Alleles↗
PubMed1981

Mitotic processes which restore genome balance in Aspergillus nidulans.

Previous work had shown that haploid strains of Aspergillus nidulans with a duplicate chromosome segment (one in normal position, one translocated to another chromosome) were unstable at mitosis; genome balance was restored by spontaneous deletion of either duplicate segment. Diploids with an extra, translocated segment showed high instability which was confined to the excess segments; loss of one of these, usually that in translocated position, gave balanced diploid nuclei and the loss was assumed to be by deletion. This led to the proposal that high-frequency deletion was provoked by, and confined to, the excess segment. In the present work it has been shown that elimination of the translocated segment in such diploids occurs more frequently by mitotic crossing over than by deletion. Accordingly, in a more rigorous test of the possible association of excess segments and deletions, a diploid homozygous for an extra, translocated segment has been studied as mitotic crossing over in this strain could not give a balanced genome. The strain was extremely unstable and gave variants of which most had a balanced, or near-balanced, diploid genome. Some variants arose by simultaneous deletions involving both non-translocated segments; almost all variants had deletions with breakpoints different from those most frequent in the corresponding, duplication haploid. The results have shown the diversity of mechanisms available for the correction of genome imbalance and that, at least in the case of Dp(I,II), the degree and modalities of mitotic instability are functions of the balance of chromosome segments and of ploidy.

Aspergillus nidulans↗
PubMed1980

Photophysical properties of zinc and magnesium tris(pyrochlorophyllide a) 1,1,1-tris(hydroxymethyl)ethane triesters.

The zinc and magnesium tris(pyrochlorophyllide a) 1,1,1-tris(hydroxymethyl)ethane triesters have properties that are very similar to those exhibited by pairs of pyrochlorophyll a or chlorophyll a linked by ethylene glycol through their propionic acid side chains. In the open configuration, the fluorescence lifetimes and emission spectra of the triesters resemble those of chlorophyll a monomers. However, the triesters have a mechanism for energy dissipation not present in monomeric chlorophyll a, which prevents the build-up of significant concentrations of molecules in S(1) states. Optical pumping of these compounds in their folded configuration results in the formation of high concentrations of molecules in S(1) states and in laser light emission, which originates from the two folded macrocycles. The optical data indicate that efficient energy transfer occurs from the open to the folded configurations.

Journal Article↗
PubMed1978

The effects of coumarin on the frequency of deletions in a duplication strain of Aspergillus nidulans.

Strains of A. nidulans with a chromosome segment in duplicate show instability resulting from deletions in either of the duplicate segments. In Dp (I, II) strains, with the terminal segment of IR attached terminally to IIR, spontaneous deletions occur most frequently, though not exclusively, from the translocated segment. Coumarin, at concentrations which did not affect viability viability or growth rate, enhanced the instability of Dp (I, II) strains by selectively increasing only the deletion class of highest spontaneous frequency. This selective action is interpreted tentatively as due to inhibition of the repair of a particular class of DNA lesion occurring spontaneously in the attachment region of Dp (I, II) strains.

Aspergillus nidulans↗
PubMed1975

Genetic control of chromosome instability in Aspergillus nidulans as a mean for gene amplification in eukaryotic microorganisms.

A haploid strain of Aspergillus nidulans carrying I-II duplication homozygous for the leaky mutation adE20 shows impreved growth on minimal medium. The duplication, though more stable than disomics, still shows instability. Several methods were used for detecting genetic control of improved stability. (a) visual selection, using a duplicated strain which is very unstable due to UV sensitivity, (adE20, biAl/dp yA2; uvsB). One stable strain showed a deletion (or a lethal mutation?) DISTAL TO BIA on the segment at the original position (on chromosome I). This deletion reduces crossing over frequency between the two homologous segments. As the deletion of the non-translocated segment (yelow sectors) must be preceded by crossing over, the above reduces the frequency of yellow sectors. A deletion of the translocated segment (green sectors) results in non-viability due to the deletion, and such sectors do not appear. The net result is a stable duplication involving only 12 C.O. units carrying the gene in concern. (b) Suppressors of UV sensitivity (su-uvsB) were attempted using the above uvs duplicated strain. Phenotypic revertants were easily obtained, but all were back mutations at the uvsB locus. (c) Mutations for UV resistance higher than that of the wild type were not obtained, in spite of the strong selective pressure inserted. (d) Recombination deficient mutations (rec), six altogether, all uvs+, did not have any effect on stability.

Adenine↗