Plasmid DNA of the recombinant plasmid

In this recombinant DNA experiment, how many hosts were utilized to obtain the end-product recombinant protein? A 700-base pair transcript encoding an enzyme was cloned into a plasmid DNA of the recombinant plasmid and was transformed into competent E.coli cells. To obtain functional recombinant enzymes, purified DNA from E.coli was subcloned into yeast from which high levels of expressed functional recombinant enzymes were obtained. Also, why was it necessary to subclone this fragment into yeast to obtain a functional recombinant protein?