Light-dependent and asynchronous replication of cyanobacterial multi-copy chromosomes

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Cyanobacteria exhibit unique multi-copy genome structures, differing from bacteria like E. coli and B. subtilis.
• The precise mechanisms governing cyanobacterial DNA replication, particularly origin identification and regulation, remain largely unelucidated.

Purpose of the Study:

• To identify the replication origin (ori) in Synechococcus elongatus PCC 7942.
• To characterize the form and synchrony of multi-copy genome replication in this freshwater cyanobacterium.

Main Methods:

• Construction of a BrdU-incorporating strain (S. 7942TK) for analyzing de novo DNA synthesis.
• Next-generation sequencing for mapping nascent DNA fragments and identifying replication origins.
• Quantitative analysis of BrdU-labeled DNA and whole-genome sequencing.

Main Results:

• Replication initiates bidirectionally from a single origin located upstream of the dnaN gene.
• DNA replication peaks before cell division and occurs asynchronously among cell populations and multi-copy chromosomes.
• BrdU uptake is light-dependent, ceasing in the dark and resuming in light.

Conclusions:

• Synechococcus elongatus PCC 7942 has a single replication origin for its multi-copy chromosomes.
• Replication initiation in S. 7942 is regulated less stringently compared to E. coli and B. subtilis.
• Asynchronous replication timing suggests unique regulatory strategies in cyanobacteria.