A novel SfaNI-like restriction-modification system in Caldicellulosiruptor extents the genetic engineering toolbox for this genus.

Abstract

Caldicellulosiruptor is a genus of thermophilic to hyper-thermophilic microorganisms that express and secrete an arsenal of enzymes degrading lignocellulosic biomasses into fermentable sugars. Because of this distinguished feature, strains of Caldicellulosiruptor have been considered as promising candidates for consolidated bioprocessing. Although a few Caldicellulosiruptor strains with industrially relevant characteristics have been isolated to date, it is apparent that further improvement of the strains is essential for industrial application. The earlier identification of the HaeIII-like restriction-modification system in C. bescii strain DSM 6725 has formed the basis for genetic methods with the aim to improve the strain's lignocellulolytic activity and ethanol production. In this study, a novel SfaNI-like restriction-modification system was identified in Caldicellulosiruptor sp. strain BluCon085, consisting of an endonuclease and two methyltransferases that recognize the reverse-complement sequences 5'-GATGC-3' and 5'-GCATC-3'. Methylation of the adenine in both sequences leads to an asymmetric methylation pattern in the genomic DNA of strain BluCon085. Proteins with high percentage of identity to the endonuclease and two methyltransferases were identified in the genomes of C. saccharolyticus strain DSM 8903, C. naganoensis strain DSM 8991, C. changbaiensis strain DSM 26941 and Caldicellulosiruptor sp. strain F32, suggesting that a similar restriction-modification system may be active also in these strains and respective species. We show that methylation of plasmid and linear DNA by the identified methyltransferases, obtained by heterologous expression in Escherichia coli, is sufficient for successful transformation of Caldicellulosiruptor sp. strain DIB 104C. The genetic engineering toolbox developed in this study forms the basis for rational strain improvement of strain BluCon085, a derivative from strain DIB 104C with exceptionally high L-lactic acid production. The toolbox may also work for other species of the genus Caldicellulosiruptor that have so far not been genetically tractable.