Genetic Transformation of Geobacillus kaustophilus HTA426 by Conjugative Transfer of Host-Mimicking Plasmids

We established an efficient transformation method for thermophile Geobacillus kaustophilus HTA426 using conjugative transfer from Escherichia coli of host-mimicking plasmids that imitate DNA methylation of strain HTA426 to circumvent its DNA restriction barriers. Two conjugative plasmids, pSTE33T and pUCG18T, capable of shuttling between E. coli and Geobacillus spp., were constructed. The plasmids were first introduced into E. coli BR408, which expressed one inherent DNA methylase gene (dam) and two heterologous methylase genes from strain HTA426 (GK1380-GK1381 and GK0343-GK0344). The plasmids were then directly transferred from E. coli cells to strain HTA426 by conjugative transfer using pUB307 or pRK2013 as a helper plasmid. pUCG18T was introduced very efficiently (transfer efficiency, $10^{-5}-10^{-3}\;recipient^{-1}$). pSTE33T showed lower efficiency ($10^{-7}-10^{-6}\;recipient^{-1}$) but had a high copy number and high segregational stability. Methylase genes in the donor substantially affected the transfer efficiency, demonstrating that the host-mimicking strategy contributes to efficient transformation. The transformation method, along with the two distinguishing plasmids, increases the potential of G. kaustophilus HTA426 as a thermophilic host to be used in various applications and as a model for biological studies of this genus. Our results also demonstrate that conjugative transfer is a promising approach for introducing exogenous DNA into thermophiles.