Complete Genome Sequence And Comparative Analysis Of Bacillus Thuringiensis Strain YBT-1520

In this thesis,we reported the whole genome sequence of Bacillus thuringiensis strain YBT-1520,which is highly toxic to lepidopteran pests and performed comparative analysis with the genomes of Bacillus anthracis Ames,Bacillus cereus ATCC 14579 and Bacillus thuringiensis 97-27.Comparison of these genomes and their plasmids reveals the similarities and differences in terms of conservation,virulence,mobile elements,metabolic competence and regulatory mechanisms.The genome of YBT-1520 is composed of a chromosome of 5,547,282bp and nine plasmids of total length of 477,287 bp,harbouring 5440 and 523 open reading frames (ORF),respectively.Of the 9 plasmids,the largest is 293,574 bp and harbors 298 ORFs, the smallest one is 2062 bp and harbors 30RFs.The analysis of orthologous genes reveals high conservation of genome content among YBT-1520,Bacillus anthracis Ames,Bacillus cereus ATCC 14579 and Bacillus thuringiensis 97-27,suggesting that they could have been inherited from a common ancestor.The chromosomally encoded virulence factors in YBT-1520 are common to the B.cereus group of bacteria,including enterotoxin, phospholipase,immune inhibitor A,Enhancin protein and chitinase,except that the ZwA gene cluster is specific to YBT-1520.Those virulence factors may enhance insecticidal toxicity to pests.Compared with Bacillus anthracis and Bacillus cereus,YBT-1520 encods more phages,IS elements and transposons,which may account for the genome plasticity and genetic exchange between the plasmid and chromosome.The YBT-1520 chromosome encods 32 putative two-component regulatory systems(5 genes involved in pathogenicity), 59 genes regulated by the pleiotropic regulator PlcR(8 genes involved in pathogenicity) and 310 genes with regulator motifs.In metabolic competence,YBT-1520 has a reduced capacity for sugar utilization and the polysaccharides which can be degraded is limited to glycogen and starch,chitin and chitosan.It lacks mannose,arabinose and rhamnose pathways and also lacks the key enzymes of inositol degradation pathway.In contrast, YBT-1520 encods abundant proteolytic enzymes,peptide and amino acid transporters and enzymes for amino acid metabolism,which is common to the B.cereus group,suggesting that their ancestor should be an opportunistic insect pathogen rather than a benign soil bacterium.We also performed analysis of the specific genes for YBT-1520,most of which are associated with replication and repair,transcription and amino acid metabolism.There is a urease operon specific for YBT-1520,which may be related to its adaptation to the protein-rich environment.The most important plasmid pBMB293 harbors a pathogenicity island consisting of five insecticidal crystal proteins(Cry1Aa,Cry1Ia,Cry2Aa andCry2Ab),one vegetative insecticidal protein(Vip3A)and one haemolytic enterotoxin operon.Comparative analysis identified a high conserved core region among pBMB293,pb10987 and pPER272, suggesting that these plasmids might have evolved from a common ancestral plasmid and formed a unique plasmid family,pBMB293 carries 12 genes related to regulation, including 2 Rap-Phr regulators for the sporulation process.There are 2 putative PlcR-binding motifs,which harbor downstream of the enterotoxin operon and the conserved hypothetical protein ORF8.Plasmid pBMB137 encodes a MerR regulator responding to environmental stimuli.The IS,such as Tn4430,Tn5044 and IS homolog to Geobacillus thermodenitrificans NG80-2p facilitate the genetic exchange for pBMB137. Plasmid pBMB137 encodes several genes involved in conjugation.The presence of nine plasmids could account for the genome plasticity and high pathogenicity for YBT-1520.