A single GAL4 dimer can maximally activate transcription under physiological conditions.

Most eukaryotic promoters contain multiple binding sites for one or more transcriptional activators that interact in a synergistic manner. A common view is that synergism is a manifestation of the need for many contacts between activators and the general transcription machinery that a single activator presumably cannot fulfill. In this model, various combinations of protein-protein interactions control the level of gene expression. However, we show here that under physiological conditions, a single binding site and presumably GAL4 can activate transcription to the maximum possible level in vivo. Synergistic effects in this natural system are shown to be consistent with cooperative DNA binding. These results point to DNA occupancy as the major element in fine tuning gene expression in the galactose regulon.

Inelastic electron tunneling spectroscopy of a Mn dimer

A scanning tunneling microscope can probe the inelastic spin excitations of single magnetic atoms in a surface via spin-flip assisted tunneling. A particular and intriguing case is the Mn dimer case. We show here that the existing theories for inelastic transport spectroscopy do not explain the observed spin transitions when both atoms are equally coupled to the scanning tunneling microscope tip and the substrate, the most likely experimental situation. The hyperfine coupling to the nuclear spins is shown to lead to a finite excitation amplitude, but the physical mechanism leading to the large inelastic signal observed is still unknown. We discuss some other alternatives that break the symmetry of the system and allow for larger excitation probabilities.