Understanding the maintenance of the giant linear plasmid SCP1 in antibiotic-producing Streptomyces

Mc Phillips, Leah (2025) Understanding the maintenance of the giant linear plasmid SCP1 in antibiotic-producing Streptomyces. Doctoral thesis, University of East Anglia.

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Abstract

Plasmid-mediated horizontal gene transfer has played a critical role in shaping bacterial genomes and indeed the world, being the driving force of the antimicrobial resistance crisis humanity faces today. Stable low-copy-number plasmid inheritance requires plasmid maintenance mechanisms, such as partition systems, to ensure plasmid retention across host generations. Whilst plasmid maintenance in unicellular bacteria is well characterised, the maintenance of plasmids in bacteria with complex multicellular life cycles, such as Streptomyces species, is poorly understood. The large linear plasmid SCP1 from Streptomyces coelicolor A3(2) encodes an antibiotic biosynthetic gene cluster and unusually is predicted to encode two partition systems, a feature previously only associated with virulence plasmids. Using bioinformatics, I demonstrate that the type Ia partition (ParABS) system is the predominant plasmid partition system type and plasmids encoding multiple partition systems are widespread, occurring across diverse bacterial taxa. However, my sequence analysis of Streptomyces plasmid ParABS systems revealed that their ParB proteins lack key residues thought to be essential for CTP-binding and hydrolysis, which are fundamental processes in canonical ParB function, questioning their role in Streptomyces plasmid maintenance. Through a combination of in vivo and biochemical techniques, I show that the SCP1 ParB proteins behave like canonical ParB proteins despite these substitutions; each bound to their distinct parS sites, exhibited characteristic ParB "spreading” along DNA, and were CTPases. However, only the ParA1B1S1 system was critical for SCP1 maintenance, whilst the ParA2B2S2 system did not play a significant role in stability. In addition, the putative conjugation protein, TraA, was also important for SCP1 stability, revealing an interdependence of plasmid segregation and conjugation to ensure SCP1 maintenance throughout the complex Streptomyces life cycle. My results provide new insights into plasmid maintenance in Streptomyces species and enhance our understanding of how antibiotic biosynthetic gene clusters and resistance genes have spread throughout bacterial populations.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Chris White
Date Deposited:	28 Jan 2026 15:15
Last Modified:	28 Jan 2026 15:15
URI:	https://ueaeprints.uea.ac.uk/id/eprint/101773
DOI:

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