ParA-like protein influences the distribution of multi-copy chromosomes in cyanobacterium Synechococcus elongatus PCC 7942

Role of Orphan ParA Proteins in Replication and Cell Division in Rhodococcus erythropolis PR4

Bacteria have a very well-regulated mechanism for chromosome segregation and cell division. This process requires a large number of complex proteins to participate and mediate their functionality. Among these complex proteins, ParA and ParB play a vital role for the faithful segregation of chromosome. In Rhodococcus erythropolis PR4, besides the essential parAB operon, there are three orphan copies of parA genes. Here, we report that the orphan ParA2 and ParA3 have distinct roles in the cell cycle. The disruption of the orphan parA2 or parA3 gene resulted in elongated cells. Multiple septal rings and mislocalised septa were observed in ΔparA3 and ΔparA2 mutants, respectively. The subcellular localization of ParA2 revealed a distinct ring- and ribbon-like structure. On the other hand, orphan ParA3 was localized slightly away from the poles. The orphan ParA proteins were found to interact with ParB, the strongest interaction was observed with ParA2. Further, asynchronous replication initiation was observed in ΔparA3 mutants suggesting its role in replication. This is the first report demonstrating the distinct roles of orphan parA genes from Rhodococcus.

Structural Determinants and Their Role in Cyanobacterial Morphogenesis

Cells have to erect and sustain an organized and dynamically adaptable structure for an efficient mode of operation that allows drastic morphological changes during cell growth and cell division. These manifold tasks are complied by the so-called cytoskeleton and its associated proteins. In bacteria, FtsZ and MreB, the bacterial homologs to tubulin and actin, respectively, as well as coiled-coil-rich proteins of intermediate filament (IF)-like function to fulfil these tasks. Despite generally being characterized as Gram-negative, cyanobacteria have a remarkably thick peptidoglycan layer and possess Gram-positive-specific cell division proteins such as SepF and DivIVA-like proteins, besides Gram-negative and cyanobacterial-specific cell division proteins like MinE, SepI, ZipN (Ftn2) and ZipS (Ftn6). The diversity of cellular morphologies and cell growth strategies in cyanobacteria could therefore be the result of additional unidentified structural determinants such as cytoskeletal proteins. In this article, we review the current advances in the understanding of the cyanobacterial cell shape, cell division and cell growth.

Specific binding of DnaA to the DnaA box motif in the cyanobacterium Synechococcus elongatus PCC 7942

In bacterial DNA replication, the initiator protein DnaA binds to the multiple DnaA box sequences located at oriC to facilitate the unwinding of duplex DNA strands. The cyanobacterium Synechococcus elongatus PCC 7942, which contains multiple chromosomal copies per cell, has DnaA box (like sequences around the oriC region, which is located upstream of dnaN. We previously observed the binding of DnaA around the oriC region; however, the DNA-binding specificity of DnaA to DnaA box sequences has not been examined. Here, we analyzed the binding specificity of DnaA protein to the DnaA box in S. elongatus by using bio-layer interferometry (BLI), a method for monitoring intermolecular interactions. We observed that recombinant DnaA protein recognized specifically the DnaA box sequence TTTTCCACA in vitro. In addition, DNA binding activity was significantly increased by R328H mutation of DnaA. This is the first report to characterize DnaA binding to the DnaA box sequence in cyanobacteria.

Cyanobacterial multi-copy chromosomes and their replication

While the model bacteria Escherichia coli and Bacillus subtilis harbor single chromosomes, which is known as monoploidy, some freshwater cyanobacteria contain multiple chromosome copies per cell throughout their cell cycle, which is known as polyploidy. In the model cyanobacteria Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803, chromosome copy number (ploidy) is regulated in response to growth phase and environmental factors. In S. elongatus 7942, chromosome replication is asynchronous both among cells and chromosomes. Comparative analysis of S. elongatus 7942 and S. sp. 6803 revealed a variety of DNA replication mechanisms. In this review, the current knowledge of ploidy and DNA replication mechanisms in cyanobacteria is summarized together with information on the features common with plant chloroplasts. It is worth noting that the occurrence of polyploidy and its regulation are correlated with certain cyanobacterial lifestyles and are shared between some cyanobacteria and chloroplasts.

ABBREVIATIONS

NGS: next-generation sequencing; Repli-seq: replication sequencing; BrdU: 5-bromo-2'-deoxyuridine; TK: thymidine kinase; GCSI: GC skew index; PET: photosynthetic electron transport; RET: respiration electron transport; Cyt b₆f complex: cytochrome b₆f complex; PQ: plastoquinone; PC: plastocyanin.

Direct Visualization of the Multicopy Chromosomes in Cyanobacterium Synechococcus elongatus PCC 7942

Cyanobacteria are prokaryotic organisms that carry out oxygenic photosynthesis. The fresh water cyanobacterium Synechococcus elongatus PCC 7942 is a model organism for the study of photosynthesis and gene regulation, and for biotechnological applications. Besides several freshwater cyanobacteria, S. elongatus 7942 also contains multiple chromosomal copies per cell at all stages of its cell cycle. Here, we describe a method for the direct visualization of multicopy chromosomes in S. elongatus 7942 by fluorescence in situ hybridization (FISH).