SummaryMulti-subunit SMC complexes control chromosome superstructure and promote chromosome disjunction, conceivably by actively translocating along DNA double helices. SMC subunits comprise an ABC ATPase “head” and a “hinge” dimerization domain connected by a 49 nm coiled-coil “arm.” The heads undergo ATP-dependent engagement and disengagement to drive SMC action on the chromosome. Here, we elucidate the architecture of prokaryotic Smc dimers by high-throughput cysteine cross-linking and crystallography. Co-alignment of the Smc arms tightly closes the interarm space and misaligns the Smc head domains at the end of the rod by close apposition of their ABC signature motifs. Sandwiching of ATP molecules between Smc heads requires them to substantially tilt and translate relative to each other, thereby opening up the Smc arms. We show that this mechanochemical gating reaction regulates chromosome targeting and propose a mechanism for DNA translocation based on the merging of DNA loops upon closure of Smc arms.

The CRISPR-Cas system is an adaptive and heritable immune response that destroys invading foreign nucleic acids. The effector complex of the Type III CRISPR-Cas system targets RNA and DNA in a transcription-coupled manner, but the exact mechanism of DNA targeting by this complex remains elusive. In this study, an effector Csm holocomplex derived from is reconstituted with a minimalistic combination of Csm12345, and shows RNA targeting and RNA-activated single-stranded DNA (ssDNA) targeting activities. Unexpectedly, in the absence of an RNA transcript, it cleaves ssDNA containing a sequence complementary to the bound crRNA guide region in a manner dependent on the HD domain of the Csm1 subunit. This nuclease activity is blocked by a repeat tag found in the host CRISPR loci. The specific cleavage of ssDNA without a target RNA suggests a novel ssDNA targeting mechanism of the Type III system, which could facilitate the efficient and complete degradation of foreign nucleic acids.

Maintenance of Chromosome (SMC) complexes are vital for chromosome organization. They extrude DNA loops to compact 2 meters of DNA into a micrometer-sized chromosome structure. The DNA loop extrusion process is believed to be a universal mechanism of SMC complexes for spatiotemporal chromosome organization conserved in almost all species from prokaryotes to eukaryotes. However, the molecular mechanism of DNA loop extrusion by SMC complexes is under debate; various tentative mechanistic models have been suggested, but there is no clear consensus. Here, we review the structural studies of various SMC complexes from prokaryotes to eukaryotes to understand the structurefunction relationships of SMC complexes involved in DNA loop extrusion. We introduce controversial observations of the conformations of SMC complexes based on previous reports and discuss various proposed mechanisms of DNA loop extrusion suggested by experimental observations of the conformations of diverse SMC complexes.

We report the crystal structure of PYCH_01220, a hypothetical protein in Pyrococcus yayanosii CH1. This protein is composed of two domains, named Domain A and Domain B. While Domain B is not significantly homologous to known protein structures, Domain A is structurally analogous to the C‐terminal ribonuclease domain of Escherichia coli colicin D. Domain A has a positively charged surface patch rendered by 13 basic residues, eight arginine or lysine residues of which are evolutionarily conserved. Electrophoretic mobility shift assays showed that PYCH_01220 binds to DNA, and charge‐inversion mutations on this patch negatively affect the DNA binding, suggesting that the function of PYCH_01220 might involve nucleic acid‐binding via the positively charged patch.

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