Marie-Claire Arnaud

H4 replication-dependent diacetylation and Hat1 promote S-phase chromatin assembly in vivo

This study examined the function of H3 and H4 tail domains in replication-dependent chromatin assembly. Results show distinct functions of H3 and H4 tails in nuclear import and chromatin assembly. Further investigations show that H4 diacetylation is essential but not sufficient for nuclear import, as preventing Hat1 binding impedes histone transport in nuclei.

Physiology and histology of resistance to Striga hermonthica in Sorghum bicolor var. Framida

Germination, attachment to host root and growth of Striga hermonthica (Del.) Benth. seeds and seedlings were studied in in vitro co-culture with Sorghum bicolor (L.) Moench and in pot experi- ments. Two varieties, the resistant Framida and the susceptible CK-60B, were used. Histological, mor- phological and physiological studies revealed the key stages of resistance mechanisms involved. Resistance of Framida to Striga does not occur at the germination or the attachment stages, since its roots do not support fewer Striga than does CK-60B. As Framida roots support the lowest number of young Striga stems with scale leaves, its resistance appears to occur during the establishment of a functional haustorium. Metabolite uptake by the haustorium and growth rate of the young parasite were lower on Framida roots than on CK-60B roots, even when similarly developed haustoria were compared. Furthermore, at a later stage of infestation, significant accumulation of a coloured material likely to be rich in phenolic compounds was observed in and around Framida conductive tissues, but not CK-60B tissues. These features indicate the involvement of at least three steps in development of resistance in Framida roots: the first is linked to a partial inhibition of development of the young haustorium; the second could play a role in the physiological events that decrease nutrient translocation towards the haus- toria; and the last seems to be associated with the accumulation of a coloured phenolic-like material.

Arginine Operator Binding by Heterologous and Chimeric ArgR Repressors from Escherichia coli and Bacillus stearothermophilus

Bacillus stearothermophilus ArgR binds efficiently to the Escherichia coli carAB operator, whereas the E. coli repressor binds very poorly to the argCo operator of B. stearothermophilus. In order to elucidate this contradictory behavior between ArgRs, we constructed chimeric proteins by swapping N-terminal DNA-binding and C-terminal oligomerization domains or by exchanging the linker peptide. Chimeras carrying the E. coli DNA-binding domain and the B. stearothermophilus oligomerization domain showed sequence-nonspecific rather than sequence-specific interactions with arg operators. Chimeras carrying the B. stearothermophilus DNA-binding domain and E. coli oligomerization domain exhibited a high DNA-binding affinity for the B. stearothermophilus argCo and E. coli carAB operators and repressed the reporter-gene transcription from the B. stearothermophilus PargCo control region in vitro; arginine had no effect on, and indeed even decreased, their DNA-binding affinity. With the protein array method, we showed that the wild-type B. stearothermophilus ArgR and derivatives of it containing only the exchanged linker from E. coli ArgR or carrying the B. stearothermophilus DNA-binding domain along with the linker and the alpha4 regions were able to bind argCo containing the single Arg box. This binding was weaker than binding to the two-box operator but was no longer arginine dependent. Several lines of observations indicate that the alpha4 helix in the oligomerization domain and the linker peptide can contribute to the recognition of single or double Arg boxes and therefore to the operator DNA-binding specificity in similar but not identical ArgR repressors from two distant bacteria.

Physiological Changes in a Root Hemiparasitic Angiosperm, Thesium humile (Santalaceae), before and after Attachment to the Host Plant (Triticum vulgare)

Summary Thesium humile , an obligate root hemiparasitic angiosperm, can live in an autotrophic-like way during several weeks before attachment to the host. About 6 weeks after germination, a decreasing growth rate and symptoms of senescence of the seedling are observed. Before attachment, Thesium is able to take up water and inorganic ions from the soil, but its mineral content is characterized by very high Na and low P levels. During the same period, rates of net photosynthesis and dark respiration as well as RuBPc and PEPc activities decrease continuously. The expression of a photosynthetic gene like psbA also decreases and the amount of transcripts of the rbcL gene is very low. After attachment to the host, Thesium recovers a more physiological balance by deriving inorganic ions from the host xylem sap. Increasing growth rate, photosynthetic carbon fixation, carboxylase capacities and expression of a photosynthetic gene like rbcL are observed and become fairly similar to those found in non-parasitic C 3 plants. A strong K accumulation in leaves also occurred and that might be an explanation for the observed anomalous stomatal behaviour. Before attachment, the poorly developed root system of Thesium seems unable to selectively absorb inorganic ions from the soil and to maintain a physiological mineral balance. Consequential to the very high Na and the low P contents in the seedling of the parasite, metabolic perturbations affecting also gene expression might result.

Mechanism of binding of serum response factor to serum response element.

Serum response factor (SRF) is a MADS transcription factor that binds to the CArG box sequence of the serum response element (SRE). Through its binding to CArG sequences, SRF activates several muscle‐specific genes as well as genes that respond to mitogens. The thermodynamic parameters of the interaction of core‐SRF (the 124–245 fragment of serum response factor) with specific oligonucleotides from c‐fos and desmin promoters, were determined by spectroscopy. The rotational correlation time of core‐SRF labeled with bis‐ANS showed that the protein is monomeric at low concentration (10−7 m). The titration curves for the fluorescence anisotropy of fluorescein‐labeled oligonucleotide revealed that under equilibrium conditions, the core‐SRF monomers were bound sequentially to SRE at very low concentration (10−9 m). Curve‐fitting data showed also major differences between the wild‐type sequence and the oligonucleotide sequences mutated within the CArG box. The fluorescence of the core‐SRF tyrosines was quenched by the SRE oligonucleotide. This quenching indicated that under stoichiometric conditions, core‐SRF was bound as a dimer to the wild‐type oligonucleotide, and as a monomer or a tetramer to the mutant oligonucleotides. Far‐UV CD spectra indicated that the flexibility of core‐SRF changed profoundly upon its binding to its specific target SRE. Lastly, the rotational correlation time of fluorescein‐labeled SRE revealed that formation of the specific complex was accompanied by a change in the SRE internal dynamics. These results indicated that the flexibility of the two partners is crucial for the DNA–protein interaction.