From a DNA-directed cell-free system, functional gal mRNA is obtained which directs the cell-free synthesis of the three galactose enzymes of Escherichia coli. A substantial fraction of this gal mRNA has the properties of a polycistronic messenger. Exposure to elevated temperatures in the presence or absence of magnesium ion results in pronounced changes of the capacity of this mRNA to give rise to the synthesis of the three enzymes. Depending on the conditions of the pre-treatment, the absolute amounts as well as the ratio of the three gene products synthesized can be changed. The different forms of gal messenger so obtained also exhibit different susceptibilities towards functional inactivation during the enzyme synthesis reaction. As the changes in template activity are reversible, it is concluded that the different treatments cause reversible transitions between different conformations of the gal mRNA.