Switching mating types with one arm tied

he mating type (MAT) locus on Saccharomyces cerevisiae chromosome III can recombine with the HMRa locus near the right telomere or the HML ␣ locus near the left telomere of the same chromosome. On page 361, Bressan et al. show that the current mating type of a cell determines the spatial configuration of these three loci in the nucleus, suggesting that the outcome of mating type switching is directed by the restraint or release of the left arm of chromosome III. GFP tagging of chromosomes in the nuclei of living cells showed that movement of the left arm of chromosome III is tightly constrained in MAT ␣ cells, but relatively free in MATa cells. Deleting the recombination enhancer (RE) sequence on chromosome III keeps the left arm constrained in both types of cells. RE activity requires the transcriptional activator Fkh1p, and T Bressan et al. suggest that Fkh1p competes for DNA binding with tethering factors that restrain the chromosome. In MATa cells, Fkh1p binding prevails, releasing the left arm and allowing HML ␣ to recombine with the MAT locus, whereas in MAT ␣ cells, restraint of the left arm leaves HMRa as the recombination donor. By directing cells to switch periodically to the opposite mating type, the system assures the availability of mating partners in a haploid population. ᭿ HML is less constrained in MATa cells (top) than in MAT␣ cells (bottom). Actin organizer takes pathogens for a ride nteropathogenic E. coli causes a dramatic actin reorganization in intestinal epithelia, erecting intracellular pedestals on the host cells beneath the attached bacteria. On page 407, Campellone et al. reduce this complex Beads covered with a Tir peptide drive actin poly-merization in an extract. E phenomenon to twelve amino acids, showing that clustering a small domain of the bacterial Tir protein, which is translocated to the host cell, is sufficient to induce actin rearrangement. The results highlight an interesting evolutionary convergence, and provide a simple model system for studying actin assembly. After discovering that Tir is the only E. coli component required for pedestal formation, the authors further whittled the system down to the C-terminal cytoplasmic domain of Tir. Clustering this domain at the plasma membrane causes its phosphorylation, allowing it to bind to the host protein Nck. Nck binding leads to the recruitment of N-WASP and the Arp2/3 actin nucleating complex, followed by actin pedestal formation. A 12–amino acid piece of …