Physics of the Bacterial Chromosome

Genomic DNA in bacteria is concentrated in a distinct structure, the bacterial chromosome or nucleoid [Pettijohn, D.E. (1996) The Nucleoid. In Neidhardt, F.C., Curtis III, R., Ingraham, J.L., Lin, E.C.C., Low, K.B., Magasanik, B., Reznikoff, W.S., Riley, M., Schaechter, M. and Umbarger, H.E. (eds.), Escherichia coli and Salmonella. ASM Press, Washington D.C., pp. 158–166; Trun, N.J. and Marko, J.F. (1998) Architecture of a bacterial chromosome. Am. Soc. Microbiol. Rev., 64, 276–283]. Stripped of all bound proteins and RNA being produced during transcription and then stretched, the length of the DNA is circa 1.5 mm, a factor of about a thousandfold larger than typical bacterial dimensions. Thus, bacteria, like all living organisms, face the daunting challenge of compacting their genome to fit inside the cell in such a way that the information encoded in the DNA will be accessible for gene expression and replication of the genome. In the case of bacteria, members of the prokaryote world (cells without a nucleus), natural selection resulted in three mechanisms that act together to compact the genome and produce a functional, dynamic architecture: supercoiling, macromolecular crowding, and the association of the DNA with a class of nucleoid‐associated or histone‐like proteins. I will now describe briefly these mechanisms.