regions of highly condensed DNA at the longitudinal axis. Condensation and organization Nucleoid-associated proteins (NAPs) In eukaryotes, genomic DNA is condensed in the form of a repeating array of DNA-protein particles called nucleosomes. A nucleosome consists of ~146 bp of DNA wrapped around an octameric complex of the histone proteins. Although bacteria do not have histones, they possess a group of DNA binding proteins referred to as nucleoid-associated proteins (NAPs) that are functionally analogous to histones in a broad sense. NAPs are highly abundant and constitute a significant proportion of the protein component of nucleoid. A distinctive characteristic of NAPs is their ability to bind DNA in both a specific (either sequence- or structure-specific) and non-sequence specific manner. As a result, NAPs are dual function proteins. The specific binding of NAPs is mostly involved in gene-specific transcription, DNA replication, recombination, and repair. At the peak of their abundance, the number of molecules of many NAPs is several orders of magnitude higher than the number of specific binding sites in the genome. Therefore, it is reasoned that NAPs bind to the chromosomal DNA mostly in the non-sequence specific mode and it is this mode that is crucial for chromosome compaction. It is noteworthy that so-called non-sequence specific binding of a NAP may not be completely random. There could be low-sequence specificity and or structural specificity due to sequence-dependent DNA conformation or DNA conformation created by other NAPs. Although molecular mechanisms of how NAPs condense DNA in vivo are not well understood, based on the extensive in vitro studies it appears that NAPs participate in chromosome compaction via the following mechanisms: NAPs induce and stabilize bends in DNA, thus aid in DNA condensation by reducing the persistence length. NAPs condense DNA by bridging, wrapping, and bunching that could occur between nearby DNA segments or distant DNA segments of the chromosome. Another mechanism by which NAPs participate in chromosome compaction is by constraining negative supercoils in DNA thus contributing to the topological organization of the chromosome. There are at least 12 NAPs identified in E. coli, the most extensively studied of which are HU, IHF, H-NS, and Fis. Their abundance and DNA binding properties and effect on DNA condensation and organization are summarized in the tables below. 1 Abundance (molecules/cell) data were taken from; The number in the parenthesis is micromolar concentration calculated using the following formula: (number of native functional units/Avogadro number)