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Indiana University Bloomington

Department of Biology

Faculty & Research

Faculty Profile

Stephen Bell

Photo of Stephen Bell
Research Images
Research photo by Stephen Bell

The Okazakisome The upper panel shows a model for a toolbelt of PCNA occupied by the three Okazaki fragment maturation enzymes,(DNA polymerase B1, Flap endonuclease and DNA ligase 1). The lower panels show our in vitro recosntituion of this maturation pathway (see Beattie and Bell, EMBO J. 2012 for more details).

Research photo by Stephen Bell

Structural basis of transcription The archaeal RNA polymerase bound to DNA substrate. The lower panels show RNA abundance profiles in conjunction with ChIP Seq analyses of Rpo2 and Rpo13 (RNAP subunits) across a heavily transcribed region of the chromosome (See Wojtas et al., NAR, 2012 for details).

Research photo by Stephen Bell

Localization of the ESCRT machinery to mid-cell in dividing Sulfolobus cells. Both the ATPase Vps4 (upper panel) and the ESCRT-III family member Saci_1373 (lower panel) localise to the site of ingression. ESCRT protein is in yellow, DNA is blue and the membrane is stained red. These and other data revealed that the Sulfolobus ESCRT apparatus is a central component of the cytokinetic machinery (for details see Samson et al.,Science 2008).

Professor of Biology and of Molecular and Cellular Biochemistry

IU Affiliations

Contact Information
By telephone: 812 856 2331
405A Simon Hall

Lab website

Genome, Cell & Developmental Biology
Research Areas
  • Chromatin, Chromosomes, and Genome Integrity
  • Evolution
  • Microbial Cell Biology and Environmental Responses

BSc in Molecular Biology, Glasgow University, Scotland
PhD in Molecular Genetics, Glasgow University, Scotland


EMBO Fellow
Alexander M Cruikshank Awardee
American Academy of Microbiology Fellow

Research Description

Overview and context

In the late 1970’s Carl Woese discovered that living organisms on Earth could be classified into one of three distinct domains, Eukarya, Bacteria and Archaea. In the time following that pivotal discovery, it has become apparent that, although morphologically resembling bacteria, the archaea in fact possess a number of molecular features more reminiscent of eukarya than bacteria. More specifically, the information processing pathways in archaea form a simplified version of the eukaryotic apparatus. Our work has primarily focussed on members of the genus Sulfolobus. In particular we have studied S. solfataricus and S. acidocaldarius. These species are hyperthermophilic acidophilic aerobes (they grow at 80°C and at pHs between 2 and 4). The hyperthermophilicity of the organisms is mirrored by an innate thermostability of the proteins that they encode. This greatly facilitates purification of native and recombinant proteins. There are also a growing number of genetic tools available for these species, further adding to their utility as model organisms.

DNA replication

DNA replication is a complex multi-step process involving the coordinated interplay of many proteins. During evolution, two distinct sets of cellular DNA replication proteins have evolved, one used by bacteria and a core machinery common to archaea and eukaryotes. In general the archaeal apparatus is a simplified version of that in eukaryotes, making Archaea a useful model system. We have mapped 3 origins of replication in the single chromosome of Sulfolobus and have characterized the interplay of initiator proteins with the origins. In addition, we are investigating the architecture of the replication fork assembly. In particular we are interested in the interplay between architectural and enzymatic components of the Okazaki fragment maturation machinery. Additionally, we have studied how replication termination is effected in Sulfolobus. Beyond the core replication architectures, we are interested in the twin processes of sister chromatid cohesion and chromosome dimer resolution in Sulfolobus.


As with DNA replication, the archaeal transcription apparatus is a slimmed down version of the eukaryotic machinery. The archaeal RNA polymerase is able to initiate transcription in vitro with the aid of just two general transcription factors, TBP and TFB. In collaboration with Nicola Abrescia (Bilbao) we are investigating the archaeal transcription apparatus using a combination of structural analyses and next generation sequencing technologies.

Cell Division

Cell division in Eukaryotes and most Bacteria and Archaea is dependent on proteins in the near ubiquitous FtsZ/Tubulin and MreB/Actin superfamilies.  However, the genomes of Sulfolobus and a number of other crenarchaea notably lack genes for these proteins. We have identified Sulfolobushomologs of the eukaryotic ESCRT system as key players in Sulfolobus cell division. The ESCRT proteins in eukaryotes play a diverse variety of roles including endosome sorting, membrane abscission during cytokinesis and is a system that is highjacked by viruses, including HIV and Ebola, to exit from cells. The Sulfolobus ESCRT system contains a limited subset of the eukaryal machinery. In particular, the early components of the eukaryal machinery, that define positioning of the apparatus lack clear orthologs in Sulfolobus. In our recent work, we have identified the factor responsible for recruiting Sulfolobus ESCRT-III to membranes at mid-cell.

Select Publications
Y. Xu, T. Gristwood, B. Hodgson, J.C. Trinidad, S.V. Albers and S.D. Bell. (2016) “Archaeal orthologs of Cdc45 and GINS interact form a stable complex that stimulates the helicase activity of MCM” Proc. Natl. Acad. Sci. (USA) 113, 13390-13395   [article]
L-P. Liew, Z-Y, Lim, M. Cohen, Z. Kong, L. Marjavaara, A. Chabes and S.D. Bell (2016) “Hydroxyurea-mediated cytotoxicity in the absence of ribonucleotide reductase inhibition” Cell Reports 17, 1657-1670  [article]
R.Y. Samson, P.D. Abeyrathne and S.D. Bell (2016) “Mechanism of archaeal MCM helicase recruitment to DNA replication origins” Molecular Cell, 61, 287-296      [article]
G. Cannone, Y. Xu, T.R. Beattie, S.D. Bell* and L. Spagnolo* (2014) “The architecture of the Okazakisome reveals the mechanism for recruitment of client proteins on PCNA” Biochem. J. 465, 239-245  [article]
J. Snyder, R.Y. Samson, S. Brumfield, S.D. Bell and M. Young. (2013) “Functional interplay between a virus and the ESCRT machinery in Archaea” PNAS, 110, 10783-10787  [article]
R.Y. Samson, Y Xu, C. Gadelha, T. Stone, J. Faqiri, D. Li, N. Qin, F. Pu, Y.X. Liang, Q She and S.D. Bell (2013)“Specificity and function of archaeal DNA replication initiator proteins” Cell Reports 3, 485-496  [article]
M. Wojtas, M.E. Mogni, O. Millet, S.D. Bell* and N.G .A. Abrescia* (2012) “Structural and functional analyses of the interaction of archaeal RNA polymerase with DNA” Nucleic Acids Research doi: 10.1093/nar/gks692  [article]
T.R. Beattie and S.D. Bell (2012) “Coordination of multiple enzyme activities by a single PCNA ring” EMBO J. 31, 1556-1567  [article]

I.G. Duggin, N. Dubarry and S.D. Bell (2011) “Replication termination and chromosome dimer resolution in the archaeon Sulfolobus solfataricus” EMBO J. 30, 145-153    [article]

R.Y. Samson, T. Obita P. L-G Chong, R.L. Williams and S.D. Bell (2011) “Molecular and structural basis of ESCRT-III recruitment to membranes during archaeal cell division” Molecular Cell, 41, 186-196  [article]

R.Y. Samson, T. Obita, S.M. Freund, R.L. Williams and S.D. Bell (2008) “A role for the ESCRT system in cell division in Archaea” Science 322, 1710-1713    [article]

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