Structural Maintenance of Chromosomes (SMC) proteins and their complex partners (ScpA and ScpB in many bacteria) are involved in chromosome compaction and segregation in all kinds of organisms. We employed single molecule tracking (SMT), tracking of chromosomal loci, and single molecule counting in Bacillus subtilis to show that in slow growing cells, ∼30 Smc dimers move throughout the chromosome in a constrained mode, while ∼60 ScpA and ScpB molecules travel together in a complex, but independently of the nucleoid. Even an Smc truncation that lacks the ATP binding head domains still scans the chromosome, highlighting the importance of coiled coil arm domains. When forming a complex, 10–15 Smc/ScpAB complexes become essentially immobile, moving slower than chromosomal loci. Contrarily, SMC-like protein RecN, which forms assemblies at DNA double strand breaks, moves faster than chromosome sites. In the absence of Smc, chromosome sites investigated were less mobile than in wild type cells, indicating that Smc contributes to chromosome dynamics. Thus, our data show that Smc/ScpAB clusters occur at several sites on the chromosome and contribute to chromosome movement.
Although DNA-compacting proteins have been extensively characterized in vitro, knowledge of their DNA binding dynamics in vivo is greatly lacking. We have employed single-molecule tracking to characterize the motion of the three major chromosome compaction factors in Bacillus subtilis, Smc (structural maintenance of chromosomes) proteins, topoisomerase DNA gyrase, and histone-like protein HBsu. We show that these three proteins display strikingly different patterns of interaction with DNA; while Smc displays two mobility fractions, one static and one moving through the chromosome in a constrained manner, gyrase operates as a single slow-mobility fraction, suggesting that all gyrase molecules are catalytically actively engaged in DNA binding. Conversely, bacterial histone-like protein HBsu moves through the nucleoid as a larger, slow-mobility fraction and a smaller, high-mobility fraction, with both fractions having relatively short dwell times. Turnover within the SMC complex that makes up the static fraction is shown to be important for its function in chromosome compaction. Our report reveals that chromosome compaction in bacteria can occur via fast, transient interactions in vivo, avoiding clashes with RNA and DNA polymerases. IMPORTANCE All types of cells need to compact their chromosomes containing their genomic information several-thousand-fold in order to fit into the cell. In eukaryotes, histones achieve a major degree of compaction and bind very tightly to DNA such that they need to be actively removed to allow access of polymerases to the DNA. Bacteria have evolved a basic, highly dynamic system of DNA compaction, accommodating rapid adaptability to changes in environmental conditions. We show that the Bacillus subtilis histone-like protein HBsu exchanges on DNA on a millisecond scale and moves through the entire nucleoid containing the genome as a slow-mobility fraction and a dynamic fraction, both having short dwell times. Thus, HBsu achieves compaction via short and transient DNA binding, thereby allowing rapid access of DNA replication or transcription factors to DNA. Topoisomerase gyrase and B. subtilis Smc show different interactions with DNA in vivo, displaying continuous loading or unloading from DNA, or using two fractions, one moving through the genome and one statically bound on a time scale of minutes, respectively, revealing three different modes of DNA compaction in vivo.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.