Genetic Environment of cry1 Genes Indicates Their Common Origin

Abstract

Although in Bacillus thuringiensis the cry genes coding for the insecticidal crystal proteins are plasmid-borne and are usually associated with mobile genetic elements, several aspects related to their genomic organization, diversification, and transmission remain to be elucidated. Plasmids of B. thuringiensis and other members of the Bacillus cereus group (n = 364) deposited in GenBank were screened for the presence of cry1 genes, and their genetic environment was analyzed using a comparative bioinformatic approach. The cry1 genes were identified in 27 B. thuringiensis plasmids ranging from 64 to 761 kb, and were predominantly associated with the ori44, ori60, or double orf156/orf157 and pXO1-16/pXO1-14 replication systems. In general, the cry1 genes occur individually or as a part of an insecticidal pathogenicity island (PAI), and are preceded by genes coding for an N-acetylmuramoyl-l-alanine amidase and a putative K+(Na+)/H+ antiporter. However, except in the case of the PAI, the latter gene is disrupted by the insertion of IS231B. Similarly, numerous mobile elements were recognized in the region downstream of cry1, except for cry1I that follows cry1A in the PAI. Therefore, the cassette involving cry1 and these two genes, flanked by transposable elements, named as the cry1 cassette, was the smallest cry1-carrying genetic unit recognized in the plasmids. Conservation of the genomic environment of the cry1 genes carried by various plasmids strongly suggests a common origin, possibly from an insecticidal PAI carried by B. thuringiensis megaplasmids.

Role of plasmid plasticity and mobile genetic elements in the entomopathogen Bacillus thuringiensis serovar israelensis

“…Because most entomotoxin genes are located within or adjacent to mobile entities (Fiedoruk et al . 2017 ) and because B. thuringiensis sv. israelensis strains carry numerous extrachromosomal molecules, they will always be able to adapt and survive by overcoming environmental changes and difficulties.…”

Completed Genomic Sequence ofBacillus thuringiensisHER1410 Reveals aCry-Containing Chromosome, Two Megaplasmids, and an Integrative Plasmidial Prophage

“…Within this island, several different transposases were annotated, suggesting multiple transposition events and pointing this region as a putative insertion hotspot ( Figure 2 ). Interestingly, the HER1410 cry1Ba4 -containing island organization resembles that of other B. thuringiensis plasmids where a N-acetylmuramoyl-l-alanine amidase coding sequence (CDS), coding for a germination key enzyme, precedes the insecticidal gene ( Fiedoruk et al , 2017 ). When the HER1410 chromosomal cry1Ba4 -containing island was compared with other plasmidial cryBa1 -containing cassettes described in Fiedoruk et al (2017) , a similar genomic organization was observed ( Figure 2 ).…”

“…Interestingly, the HER1410 cry1Ba4 -containing island organization resembles that of other B. thuringiensis plasmids where a N-acetylmuramoyl-l-alanine amidase coding sequence (CDS), coding for a germination key enzyme, precedes the insecticidal gene ( Fiedoruk et al , 2017 ). When the HER1410 chromosomal cry1Ba4 -containing island was compared with other plasmidial cryBa1 -containing cassettes described in Fiedoruk et al (2017) , a similar genomic organization was observed ( Figure 2 ). Also, as in cry1Ba -containing plasmids, the HER1410 island only posseses one pesticidal gene (not within a PAI) and it is not preceded by a K þ (Na þ )/H þ antiporter.…”

“…Similarly to the toxin-encoding plasmid B. thuringiensis pBtoxis, this region may contain the replication origin of pLUSID1 and these proteins could be responsible for a self-replication system and possibly the segregation and partitioning Figure 2 HER1410 chromosomal cryBa4-containing genomic island analysis. Comparison of cry1Ba4-containing genomic island with described cry1Ba-containing plasmid cassettes from Fiedoruk et al (2017) was rendered with EasyFig and further details were added with Adobe Illustrator. Predicted protein-coding genes are indicated with arrows, indicating the direction of transcription.…”

“…These genes are usually flanked by mobile elements (insertion sequences and transposons) and can grouped with other insecticidal proteins to form pathogenicity islands (PAI). This organization and the surrounding genetic context of these genes have been described to facilitate their mobility among plasmids and strains (Fiedoruk et al, 2017).…”