Prokaryotes have a single chromosome consisting of circular double-stranded DNA. The size may vary considerably, e.g. the E. coli chromosome contains about 3.8×106 bp, while that of Bacillus subtilis has 2×106 bp and that of Salmonella typhimurum has 10.5×106 bp. If the E. coli chromosome were in a linear extended form it would be about 1 mm long, but it is a fairly compact structure due to supercoiling of the DNA. This must be opened up to allow access for enzymes involved in replication or transcription in complex processes requiring several proteins, including helicases that relax supercoiled DNA, and single-strand binding protein (SSBP) (Chapter 1.3). Other proteins that are involved in these processes were originally identified by genetic means and are named after the genes that encode them (e.g. dnaB, dnaC, for the genes; DnaB, DnaC for the proteins).

Replication

This has been studied in simplified systems with the DNA from phages like ϕX174 and the plasmid pBR 322 that have specific replication origins. A multisubunit complex of primase (Pri) proteins is built up on these sites, known as primosome assembly sites. PriA binds to a single-stranded hairpin structure and acts as a 3′ → 5′ helicase, translocating along double-stranded DNA with the concomitant hydrolysis of ATP, and PriB is bound to this complex.