Abstract
Geobacillus kaustophilus HTA426 is a thermophilic bacterium whose genome harbors numerous insertion sequences (IS). This study was initially conducted to generate mutant genes for thermostable T7 RNA polymerase in G. kaustophilus; however, relevant experiments unexpectedly identified that the organism transposed multiple IS elements and produced derivative cells that expressed a silent gene via transposition. The transposed elements were diverse and included members of the IS4, IS701, IS1634, and ISLre2 families. The transposition was relatively active at elevated temperatures and generated 4–9 bp of direct repeats at insertion sites. Transposition was more frequent in proliferative cells than in stationary cells but was comparable between both cells when sigX, which encodes an extra-cytoplasmic function sigma factor, was forcibly expressed. Southern blot analysis indicated that IS transposition occurred under growth inhibitory conditions by diverse stressors; however, IS transposition was not detected in cells that were cultured under growth non-inhibitory conditions. These observations suggest that G. kaustophilus enhances IS transposition via sigX-dependent stress responses when proliferative cells were prevented from active propagation. Considering Geobacillus spp. are highly adaptive bacteria that are remarkably distributed in diverse niches, it is possible that these organisms employ IS transposition for environmental adaptation via genetic diversification. Thus, this study provides new insights into adaptation strategies of Geobacillus spp. along with implications for strong codependence between mobile genetic elements and highly adaptive bacteria for stable persistence and evolutionary diversification, respectively. This is also the first report to reveal active IS elements at elevated temperatures in thermophiles and to suggest a sigma factor that governs IS transposition.
Highlights
Insertion sequences (IS) are a simple class of mobile genetic elements that propagate themselves or change the position in the host’s genetic material via replicative or non-replicative transposition, respectively (Vandecraen et al, 2018)
G. kaustophilus MK536 is auxotrophic for uracil; this strain unexpectedly produced uracil prototrophs when incubated on minimum medium without uracil (MU)
It was hypothesized that PT7 mutations were responsible for T7 RNA polymerase (T7RP)-independent pyrF expression because uracil prototrophs were not generated from G. kaustophilus MK242; we sequenced upstream of pyrF in 49 prototrophs including G. kaustophilus MK536up1
Summary
Insertion sequences (IS) are a simple class of mobile genetic elements that propagate themselves or change the position in the host’s genetic material via replicative or non-replicative transposition, respectively (Vandecraen et al, 2018). An IS element is flanked by short inverted repeats and encodes a transposase that catalyzes transposition. Transposition potentially causes deleterious mutations; IS elements were initially considered parasitic and selfish factors that multiply without conferring a survival advantage to the host organism (Schrader and Schmitz, 2019). It is known that IS transposition can provide evolutionary adaptation for their hosts via gene inactivation and/or modulated expression of the neighboring genes (Vandecraen et al, 2018)
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