Nucleoid

The nucleoid is a distinct, membrane-free region in prokaryotic cells that contains the cell’s DNA and associated proteins.

In most bacteria and archaea, the DNA is a single, circular, double-stranded molecule highly compacted through supercoiling and the action of architectural proteins called nucleoid-associated proteins or NAPs.

These proteins help organize DNA into chromosome interaction domains, ensuring structural stability, and making it accessible for replication, transcription, and DNA repair.

NAPs are also crucial for cell division by helping segregate the chromosomes into daughter cells.

In archaea Sulfolobus, the chromosome occupies two distinct compartments, with high and low gene expression regions. Coalescin, an archaeal-specific NAP, helps maintain these distinct compartments, ensuring expression of genes based on the cell’s needs.

Many archaea use histone-like proteins as NAPs to organize their DNA into nucleosome-like structures similar to those in eukaryotes.

However, these histones can form tetramers or larger complexes, differing from the octameric structure in eukaryotic cells.