Unique Function of the Bacterial Chromosome Segregation Machinery in Apically Growing Streptomyces - Targeting the Chromosome to New Hyphal Tubes and its Anchorage at the Tips

Abstract

The coordination of chromosome segregation with cell growth is fundamental to the proliferation of any organism. In most unicellular bacteria, chromosome segregation is strictly coordinated with cell division and involves ParA that moves the ParB nucleoprotein complexes bi- or unidirectionally toward the cell pole(s). However, the chromosome organization in multiploid, apically extending and branching Streptomyces hyphae challenges the known mechanisms of bacterial chromosome segregation. The complex Streptomyces life cycle involves two stages: vegetative growth and sporulation. In the latter stage, multiple cell divisions accompanied by chromosome compaction and ParAB assisted segregation turn multigenomic hyphal cell into a chain of unigenomic spores. However, the requirement for active chromosome segregation is unclear in the absence of canonical cell division during vegetative growth except in the process of branch formation. The mechanism by which chromosomes are targeted to new hyphae in streptomycete vegetative growth has remained unknown until now. Here, we address the question of whether active chromosome segregation occurs at this stage. Applied for the first time in Streptomyces, labelling of the chromosomal replication initiation region (oriC) and time-lapse microscopy, revealed that in vegetative hyphae every copy of the chromosome is complexed with ParB, whereas ParA, through interaction with the apical protein complex (polarisome), tightly anchors only one chromosome at the hyphal tip. The anchor is maintained during replication, when ParA captures one of the daughter oriCs. During spore germination and branching, ParA targets one of the multiple chromosomal copies to the new hyphal tip, enabling efficient elongation of hyphal tube. Thus, our studies reveal a novel role for ParAB proteins during hyphal tip establishment and extension.

Bacterial Cell Division: Nonmodels Poised to Take the Spotlight

“…SsgB, the protein that brings FtsZ to the division sites during sporulation, mislocalizes in the absence SepG, a transmembrane protein that may be involved in nucleoid compaction (144), suggesting that chromosome organization and cell division may be coordinately regulated during sporulation in S. coelicolor . Consistent with this notion, the chromosome segregation proteins ParAB in this organism is able to regulate cell tip elongation and FtsZ assembly (30, 76), and the broadly conserved multi-functional cell division protein DivIVA interacts with ParA at least indirectly via a cytoskeletal protein Scy (29, 62). Finally, it is worth noting that, although most of the cell division research in S. coelicolor has been focused on sporulation in this organism due to the dispensability of ftsZ during normal growth, new observations suggest that this naturally filamentous bacterium does indeed routinely separate its cytosol and achieve compartmentalization by formation of FtsZ-independent “cross-membranes” that represent a form of cell division whose molecular details of formation and regulation await further study (23, 143).…”

Competition between DivIVA and the nucleoid for ParA binding promotes segrosome separation and modulates mycobacterial cell elongation

“…Chromosome segregation and oriC positioning also depend on the interaction of chromosome segregation proteins with other protein complexes. A growing body of evidence shows that in various bacterial species ( C. crescentus, V. cholerae, C. glutamicum, S. coelicolor and M. xanthus ), the oriC region is anchored at the cell pole or subpolarly due to interactions between ParA or ParB and polar or subpolar proteins (Bowman et al , ; Ebersbach et al , ; Donovan et al , ; Yamaichi et al , ; Treuner‐Lange and Søgaard‐Andersen, ; Kois‐Ostrowska et al , ). In C. crescentus , the best‐studied cell cycle model species, segregation proteins interact with the pole‐associated proteins PopZ and TipN (Bowman et al , ; Ebersbach et al et al , ), while in V. cholerae polar HubP protein (Yamaichi et al , ), controls the polar localisation of the oriC region.…”

“…In C. crescentus , the best‐studied cell cycle model species, segregation proteins interact with the pole‐associated proteins PopZ and TipN (Bowman et al , ; Ebersbach et al et al , ), while in V. cholerae polar HubP protein (Yamaichi et al , ), controls the polar localisation of the oriC region. Similarly, in hyphal S. coelicolor , the interaction between ParA and a component of the polarisome complex, the Scy protein, is required for apical attachment of oriC (Ditkowski et al , ; Kois‐Ostrowska et al , ). Interestingly, in M. xanthus ParA interaction with bactofilin scaffolds constrains the chromosome segregation machinery to the subpolar regions of the cell (Lin et al , ).…”

Reconciling DNA replication and transcription in a hyphal organism: Spatial dynamics of transcription complexes in live Streptomyces coelicolor A3(2)

“…The processes of transcription and chromosome replication both occupy the same cellular template and understanding how such conflicts are reconciled is fundamental to understanding the complexities of bacterial growth and the structure of the dynamic bacterial nucleoid 1,2,3 . In eukaryotes this problem is solved by segregating growth and replication in to separate stages within the cell cycle.…”

“…In eukaryotes this problem is solved by segregating growth and replication in to separate stages within the cell cycle. In bacteria, this is not the case and spatial organisation of the nucloids is dependent on the growth habits and morphology of the specific bacterium 1 . Bacterial RNAP is highly sensitive to environmental cues and is subject to significant compaction and expansion forces due to the action of DNA-binding proteins, DNA supercoiling, macromolecular crowding, interaction with cytoskeletal proteins and transertion 4,5 impacting on other cell processes such as DNA replication.…”