A Thermus thermophilus HB27 strain was constructed in which the malate dehy
drogenase (mdh) gene was deleted. The Delta mdh 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 introdu
ction of plasmid DNA into Thermus spp., and because mdh levels can be readi
ly quantified, this host-vector system isa convenient tool for monitoring g
ene expression.