Characterization of a tectivirus-like molecule found in the Bacillus cereus group

Details

Supervisors

Gillis, Annika

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences agronomiques, à finalité spécialisée

Abstract

The tectivirus-like lysogenic molecule pGIL16-18, found in Bacillus thuringiensis strain BMG1.7, has an 18 kb genome that is divided into two distinct segments: the initial 15 kb with the PolB1 polymerase shows high similarity to Betatectivirus GIL16, while the remaining 3 kb encodes a polymerase resembling that of Betatectivirus Wip1 (PolB2). This master dissertation aims to elucidate the characteristics of pGIL16-18, investigate the potential role of its PolB2 polymerase, and explore other alternative hosts. The results of this work indicate that pGIL16-18 predominantly forms clear plaques, in contrast to the turbid plaques generated by the GIL16 phage, suggesting that the additional 3 kb segment encoding PolB2 may influence plaque morphology. Further analysis and experimental data are needed to confirm these findings and establish a definitive link between genomic structure and phage plaque appearance. A significant discovery of this work is the ability of the 3 kb PolB2-encoding segment to excise from the 18 kb linear genome and circularize. This conclusion is supported by the presence of both 15 kb and 18 kb tectiviral-molecules in phage plaque samples. Moreover, structural predictions using AlphaFold revealed high similarity between the core domains of PolB1 and PolB2, with root-mean-square deviations (RMSD) values ranging from 1.206 Å to 1.294 Å. Notable structural differences in the exonuclease domains between these two family B DNA polymerases suggest potential variations in functional roles and proofreading capabilities. Additionally, this work identified 17 strains with molecules similar to pGIL16-18 and six new tectivirus variants. Comparative genomic analysis confirmed that both B. thuringiensis strain BMG1.7 and B. cereus s.s. strain MHI2385 serve as hosts for pGIL16-18, with B. cereus s.s. strain MHI2385 identified as a new host through whole-genome sequencing. These findings contribute to a deeper understanding of the diversity of tectiviral plasmidial prophages and, in particular, of pGIL16-18’s genomic organization, DNA polymerase putative roles, and host interactions, offering valuable insights for future research into its ecological and evolutionary impact.