The new gene mukB codes for a 177 kd protein with coiled-coil domains involved in chromosome partitioning of E. coli.

Abstract

An Escherichia coli temperature sensitive mutant which produces spontaneously normal size anucleate cells at low temperature was isolated. The mutant is defective in a previously undescribed gene, named mukB, located at 21 min on the chromosome. The mukB gene codes for a large protein (approximately 180 kd). A 1534 amino acid protein (176,826 daltons) was deduced from the nucleotide sequence of the mukB gene. Computer analysis revealed that the predicted MukB protein has distinct domains: an amino‐terminal globular domain containing a nucleotide binding sequence, a central region containing two alpha‐helical coiled‐coil domains and one globular domain, and a carboxyl‐terminal globular domain which is rich in Cys, Arg and Lys. A 180 kd protein detected in wild‐type cell extracts by electrophoresis is absent in mukB null mutants. Although the null mutants are not lethal at low temperature, the absence of MukB leads to aberrant chromosome partitioning. At high temperature the mukB null mutants cannot form colonies and many nucleoids are distributed irregularly along elongated cells. We conclude that the MukB protein is required for chromosome partitioning in E. coli.

The bacterial nucleoid: A highly organized and dynamic structure

“…The linker regions of two monomers then interact to form a V-shaped complex with a variable opening angle between 0 and 1808 [Melby et al, 1998;Hirano et al, 2001]. A similar arrangement was observed for MukB from E. coli, which might represent a highly divergent SMC homolog [Niki et al, 1991[Niki et al, , 1992Melby et al, 1998]. In the presence of DNA, ATP binding promotes end-to-end association of different SMC dimers, which initiates the formation of larger nucleoprotein complexes, but in turn also activates hydrolysis of the bound nucleotides [Hirano et al, 2001].…”

“…Both SMC and MukB were shown to cooperate with two accessory proteins, which regulate their function [Hirano and Hirano, 2004] and are essential for their activity in vivo [Yamanaka et al, 1996;Mascarenhas et al, 2002]. Inactivation of either of these complexes leads to temperaturesensitive growth due to a severe disturbance of nucleoid organization and chromosome segregation at elevated temperatures [Niki et al, 1991;Britton et al, 1998;Moriya et al, 1998;Jensen and Shapiro, 1999]. This phenotype was shown to be suppressed by mutations that cause excessive negative supercoiling, which suggests a role for these proteins in DNA compaction or folding [Sawitzke and Austin, 2000;Lindow et al, 2002a].…”

Independent Segregation of the Two Arms of theEscherichia coli oriRegion Requires neither RNA Synthesis nor MreB Dynamics

“…Therefore, the key to efficient and faithful segregation is likely to reside in chromosome organization itself and the processes that drive this organization, as well as independent replication by spatially separated replisomes tracking along the DNA and the subsequent decatenation by TopoIV. Consistent with this view, aberrant chromosome organization as a consequence of absence of functional SMC complexes (MukbEF) leads to an altered pattern of replication-segregation and to failures in chromosome segregation (8,10,44). It seems to us that the independent tracking of sister replisomes along DNA, outward from oriC (47), may facilitate the segregation of newly replicated sister chromosomes into separate cell halves, thereby allowing the entropic mechanism to mediate the segregation process efficiently.…”

MukB-mediated Catenation of DNA Is ATP and MukEF Independent

“…How then to reconcile the biochemical observations to those in vivo that indicate mukE and mukF mutations have the same phenotype as mukB mutations (33,34), and that MukB ATPbinding and MukEF are required for MukB localization in the cell (11,12)? These observations are not necessarily contradictory.…”