AT-rich region and repeated sequences - the essential elements of replication origins of bacterial replicons.

Repeated sequences are commonly present in the sites for DNA replication initiation in bacterial, archaeal, and eukaryotic replicons. Those motifs are usually the binding places for replication initiation proteins or replication regulatory factors. In prokaryotic replication origins, the most abundant repeated sequences are DnaA boxes which are the binding sites for chromosomal replication initiation protein DnaA, iterons which bind plasmid or phage DNA replication initiators, defined motifs for site-specific DNA methylation, and 13-nucleotide-long motifs of a not too well-characterized function, which are present within a specific region of replication origin containing higher than average content of adenine and thymine residues. In this review, we specify methods allowing identification of a replication origin, basing on the localization of an AT-rich region and the arrangement of the origin's structural elements. We describe the regularity of the position and structure of the AT-rich regions in bacterial chromosomes and plasmids. The importance of 13-nucleotide-long repeats present at the AT-rich region, as well as other motifs overlapping them, was pointed out to be essential for DNA replication initiation including origin opening, helicase loading and replication complex assembly. We also summarize the role of AT-rich region repeated sequences for DNA replication regulation.