The eclipse period of Escherichia coli

The minimal time between successive initiations on the same origin (the ecl ipse) in Escherichia coli was determined to be similar to 25-30 min. An inv erse relationship was found between the length of the eclipse and the amoun t of Dam methyltransferase in the cell, indicating that the eclipse corresp onds to the period of origin hemimethylation. The SeqA protein was absolute ly required for the eclipse, and DnaA titration studies suggested that the SeqA protein prevented the binding of multiple DnaA molecules on oriC (init ial complex formation). No correlation between the amount of SeqA and eclip se length was revealed, but increased SeqA levels affected chromosome parti tioning and/or cell division. This was corroborated further by an aberrant nucleoid distribution in SeqA-deficient cells. We suggest that the SeqA pro tein's role in maintaining the eclipse is tied to a function in chromosome organization.