Abstract
Our previous work established that DNA is naturally transferable on agar plates through a new transformation system which is regulated by the stationary phase master regulator RpoS in Escherichia coli. In this transformation system, neither additional Ca2+ nor heat shock is required. Instead, transformation is stimulated by agar. The membrane protein OmpA, a gated pore permeable to ions and larger solutes, serves as a receptor for DNA transfer during bacteriophage infection and conjugation. However, it remains unknown how DNA transfers across membranes and whether OmpA is involved in transformation of E. coli. Here, we explored potential roles of OmpA in natural and chemical transformation of E. coli. We observed that ompA inactivation significantly improved natural transformation on agar plates, indicating that OmpA blocks DNA transfer. Transformation promotion by ompA inactivation also occurred on soft plates, indicating that OmpA blocks DNA transfer independent of agar. By contrast, compared with the wild-type strain, chemical transformation of the ompA mutant was lower, indicating that OmpA has a role in DNA transfer. Inactivation of ompA also reduced chemical transformation in solution containing less Ca2+ or with a shortened time for heat shock, suggesting that the promotion effect of OmpA on DNA transfer does not solely rely on Ca2+ or heat shock. We conclude that OmpA plays opposite roles in natural and chemical transformation: it blocks DNA uptake on agar plates but promotes DNA transfer in the liquid Ca2+ solution. Considering that no single factor was identified to reverse the function of OmpA, we propose that multiple factors may cooperate in the functional reversal of OmpA during natural and artificial transformation of E. coli. Finally, we observed that ompA transcription was not affected by the expression of RpoS, excluding the possibility that RpoS regulates DNA transfer by suppressing ompA transcription.
Highlights
Horizontal gene transfer (HGT) provides bacteria with new genetic traits to better survive in the changing environment [1]
The multiple roles of OmpA in DNA transfer during conjugation and bacteriophage infection, as well as colicin transport, stimulated us to test its potential role in natural transformation on agar plates
We showed that OmpA played opposite roles in mediating DNA transfer during natural and chemical transformation: it blocks DNA transfer during natural transformation (Fig. 1) but boosts DNA transfer in chemical transformation of E. coli (Fig. 3)
Summary
Horizontal gene transfer (HGT) provides bacteria with new genetic traits to better survive in the changing environment [1]. Three forms of HGT exist in nature: transformation, conjugation and bacteriophage infection [3]. In both conjugation and bacteriophage infection, transferring DNA is coated with proteins which help DNA enter the recipient cells. Conjugation and bacteriophage infection have been well documented in Escherichia coli for a long time [4,5], but this species is commonly known as a non-naturally transformable bacterium because it was transformable only with artificial treatments (e.g. heat shock and a high concentration of divalent cations) [6,7,8,9]. Transformation of E. coli on agar plates without these artificial treatments has been repeatedly reported by several independent groups [10,11,12,13]
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