Set1 is the catalytic subunit and the central component of the evolutionarily conserved Set1 complex (Set1C) that methylates histone 3 lysine 4 (H3K4). Here we have determined protein/protein interactions within the complex and related the substructure to function. The loss of individual Set1C subunits differentially affects Set1 stability, complex integrity, global H3K4 methylation, and distribution of H3K4 methylation along active genes. The complex requires Set1, Swd1, and Swd3 for integrity, and Set1 amount is greatly reduced in the absence of the Swd1-Swd3 heterodimer. Bre2 and Sdc1 also form a heteromeric subunit, which requires the SET domain for interaction with the complex, and Sdc1 strongly interacts with itself. Inactivation of either Bre2 or Sdc1 has very similar effects. Neither is required for complex integrity, and their removal results in an increase of H3K4 mono-and dimethylation and a severe decrease of trimethylation at the 5 end of active coding regions but a decrease of H3K4 dimethylation at the 3 end of coding regions. Cells lacking Spp1 have a reduced amount of Set1 and retain a fraction of trimethylated H3K4, whereas cells lacking Shg1 show slightly elevated levels of both di-and trimethylation. Set1C associates with both serine 5-and serine 2-phosphorylated forms of polymerase II, indicating that the association persists to the 3 end of transcribed genes. Taken together, our results suggest that Set1C subunits stimulate Set1 catalytic activity all along active genes.

The Type VI secretion system (T6SS) is a multiprotein weapon used by bacteria to destroy competitor cells. The T6SS contractile sheath wraps an effector-loaded syringe that is injected into the target cell. This tail structure assembles onto the baseplate that is docked to the membrane complex. In entero-aggregative Escherichia coli TssA plays a central role at each stage of the T6SS assembly pathway by stabilizing the baseplate and coordinating the polymerization of the tail. Here we adapted an assay based on APEX2dependent biotinylation to identify the proximity partners of TssA in vivo. By using stage-blocking mutations, we define the temporal contacts of TssA during T6SS biogenesis. This proteomic mapping approach also revealed an additional partner of TssA, TagA. We show that TagA is a cytosolic protein tightly associated with the membrane. Analyses of sheath dynamics further demonstrate that TagA captures the distal end of the sheath to stop its polymerization and to maintain it under the extended conformation. The bacterial Type VI secretion system (T6SS) is a tail structure that uses a contractile mechanism to inject a molecular syringe loaded with effectors into target cells 1-8. This sophisticated apparatus is widespread in Gram-negative bacteria, and could be deployed to deliver effectors to the milieu, or into eukaryotic host cells or competitor bacterial cells. The T6SS helps to establish symbiosis, to collect metals or to disable or kill target cells 5,9-13. At the molecular level, the T6SS requires a minimum set of 12 proteins that are indispensable for its assembly and function whereas additional, accessory proteins such as peptidoglycan