GLOBOSA (GLO) is a homeotic gene whose mutants show sepaloid petals and carpelloid stamens. The similarity of Glo mutants to those of the DEFICIENS (DEFA) gene suggests that the two genes have comparable functions in floral morphogenesis. The GLO cDNA has been cloned by virtue of its homology to the MADS‐box, a conserved DNA‐binding domain also contained in the DEFA gene. We have determined the structure of the wild type GLO gene as well as of several glo mutant alleles which contain transposable element insertions responsible for somatic and germinal instability of Glo mutants. Analyses of the temporal and spatial expression patterns of the DEFA and GLO genes during development of wild type flowers and in flowers of various stable and unstable defA and glo alleles indicate independent induction of DEFA and GLO transcription. In contrast, organ‐specific up‐regulation of the two genes in petals and stamens depends on expression of both DEFA and GLO. In vitro DNA‐binding studies were used to demonstrate that the DEFA and GLO proteins specifically bind, as a heterodimer, to motifs in the promoters of both genes. A model is presented which proposes both combinatorial and cross‐regulatory interactions between the DEFA and GLO genes during petal and stamen organogenesis in the second and third whorls of the flower. The function of the two genes controlling determinate growth of the floral meristem is also discussed.
A population of a mutT strain of E. coli was maintained in a chemostat for 2,200 generations. Afterwards the rate, of mutation to resistance to three antibiotics was determined by the Luria-Delbrück fluctuation test. It was found that the strain had a distinctly reduced mutability after the long-term cultivation compared with the original strain. Nevertheless the mutability was still much higher than that of a wild-type strain. After transduction of the mutT gene into another genetic background the transductants showed the same mutability as the original strain indicating that the mutT allele itself had not changed. Our results support the hypothesis that under new environmental conditions mutator strains have an advantage due to their more efficient production of beneficial mutations. After optimal adaptation there is selection against high mutation rates due to the increased mutational load in the mutator population.
Previous studies have shown that the mutT, mutH and mutL mutators of Escherichia coli have a marked advantage in competition growth with otherwise coisogenic wild-type strains. As shown in this paper the same is true for the mutS mismatch mutator. In three experiments mutS could outgrow the wild-type and had higher fitness values.
Competition experiments between Escherichia coli mutL and mut+ populations show that the mutator gene confers a selective advantage on the strain which carries it. Fitness values vary from experiment to experiment.
Flowers of the temperature-sensitive DEFICIENS (DEF) mutant, def-101, display sepaloid petals and carpelloid stamens when grown at 26 degrees C, the non-permissive temperature. In contrast, when cultivated under permissive conditions at 15 degrees C, the morphology of def-101 flowers resembles that of the wild type. Temperature shift experiments during early and late phases of flower development revealed that second and third whorl organ development is differentially sensitive to changes in DEF expression. In addition, early DEF expression seems to control the spatially correct initiation of fourth whorl organ development. Reduction of the def-101 gene dosage differentially affects organogenesis in adjacent whorls: at the lower temperature development of petals in the second whorl and initiation of carpels in the centre of the flower is not affected while third whorl organogenesis follows the mutant (carpelloid) pattern. The possible contribution of accessory factors to organ-specific DEF functions is discussed. In situ analyses of mRNA and protein expression patterns during def-101 flower development at 15 degrees C and at 26 degrees C support previously proposed combinatorial regulatory interactions between the MADS-box proteins DEF and GLOBOSA (GLO), and provide evidence that the autoregulatory control of DEF and GLO expression by the DEF/GLO heterodimer starts after initiation of all organ primordia. Immunolocalisation revealed that both proteins are located in the nucleus. Interestingly, higher growth temperature affects the stability of both the DEF-101 and GLO proteins in vivo. In vitro DNA binding studies suggest that the temperature sensitivity of the def-101 mutant is due to an altered heterodimerisation/DNA-binding capability of the DEF-101 protein, conditioned by the deletion of one amino acid within the K-box, a protein region thought to be involved in protein-protein interaction. In addition, we introduce a mutant allele of GLO, glo-confusa, where insertion of one amino acid impairs the hydrophobic carboxy-terminal region of the MADS-box, but which confers no strong phenotypic changes to the flower. The strong mutant phenotype of flowers of def-101/glo-conf double mutants when grown in the cold represents genetic evidence for heterodimerisation between DEF and GLO in vivo. The potential to dissect structural and functional domains of MADS-box transcription factors is discussed.
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