New host-vector system for Thermus spp. based on the malate dehydrogenase gene

A Thermus thermophilus HB27 strain was constructed in which the malate dehydrogenase (mdh) gene was deleted. The Deltamdh colonies are recognized by a small-colony phenotype. Wild-type phenotype is restored by transformation with Thermus plasmids or integration vector containing an intact mdh gene. The wild-type phenotype provides a positive selection tool for the introduction of plasmid DNA into Thermus spp., and because mdh levels can be readily quantified, this host-vector system is a convenient tool for monitoring gene expression.