Journal of Theoretical Biology 225 (2003) 493–496 DNA–membrane interactions can localize bacterial cell center Avinoam Rabinovitch a, *, Arieh Zaritsky b , Mario Feingold a a Departments of Physics, Ben-Gurion University of the Negev, P.O. Box. 653, Be’er-Sheva 84105, Israel b Departments of Life Sciences, Ben-Gurion University of the Negev, P.O. Box. 653, Be’er-Sheva 84105, Israel Received 19 November 2002; received in revised form 14 May 2003; accepted 18 July 2003 Dedicated to AZ’s parents, Bezal’el and Rachel Zaritsky, who passed away in their mid-90s during the summer of 2002 Abstract In actively growing bacterial cells, the DNA exerts stress on the membrane, in addition to the turgor caused by osmotic pressure. This stress is applied through coupled transcription/translation and insertion of membrane proteins (so-called ‘‘transertion’’ process). In bacillary bacteria, the strength of this interaction varies along cell length with a minimum at its midpoint, and hence can locate the cell’s equator for the assembly of the FtsZ-ring. r 2003 Elsevier Ltd. All rights reserved. Keywords: Symmetry in cell division; Tug of war; FtsZ-ring assembly; Nucleoid occlusion 1. Introduction In cell division of rod-shaped bacteria, the asymmetry between daughters is exceedingly low (coefficient of variation (CV) of sister cell lengths can be as small as 4%) (Errington et al., 1965; Burdett and Higgins, 1978; Koppes et al., 1978; Trueba, 1981; Grover and Woldringh, 2001). This precision is surprising since CV values of the other major cell-cycle events (cell size/ age at birth, initiation and termination of chromosome replication and rate of elongation) leading to division are larger (Schaechter et al., 1962; Boye et al., 1996). The mechanism that determines the exact cell length midpoint has been debated during the last four decades (Koch and Schaechter, 1962; Koch, 1977) but is still unresolved. Several models have been proposed to explain this enigma (see e.g. Cooper, 1991; Koch, 2000), none of which seems to us satisfactory. The model currently in vogue relies on the oscillations of the MinCDE system (Raskin and de Boer, 1996; Meinhardt and de Boer, 2001; Kruse, 2002). According to this model, the oscillations of these proteins identify the cell midpoint by the difference in time spent there rather than elsewhere. A tempting mechanism for center location is based on the so-called ‘‘Tug Of War’’ (TOW) concept (Koch and Holtje, 1995). This process is analogous to two teams pulling at two sides of a rope leading to a stress maximum at the midpoint. The bacterium would find its midpoint if it could discern stress levels. During growth, the peptidoglycan pulls the cytoplasmic membrane (CM) causing an uneven stress level in the CM. The mechanism proposed by Koch and Holtje (1995) relies on a delicate imbalance between the growth rates of the peptidoglycan and the CM. Since information about these rates is incomplete and other unverified assump- tions are necessary for their model, it is yet to be tested. Nevertheless, we suggest that the TOW concept could be validated by the use of the interactions between the membrane and the bacterial chromosome. 2. DNA–membrane interactions The nucleoid shapes adopted by the chromosome differ in different physiological conditions (Woldringh, 1976; Zaritsky et al., 1999, 2000). In actively growing and dividing cells, it is diffuse in form, occupying a large portion of the cytoplasm and connected to the CM, while in resting cells it is compacted in the center (Morgan et al., 1967; Van Helvoort et al., 1998). This transformation is explained by a change in the balance ARTICLE IN PRESS *Corresponding author. Tel.: +972-8-646-1172; fax: +972-8-647- 2904. E-mail address: avinoam@bgumail.bgu.ac.il (A. Rabinovitch). 0022-5193/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-5193(03)00292-3