The present invention relates to polynucleotide molecules comprising
nucleotide sequences encoding an aveC gene product, which polynucleotide
molecules can be used to alter the ratio or amount of class 2:1
avermectins produced in fermentation cultures of S. avermitilis. The
present invention further relates to vectors, host cells, and mutant
strains of S. avermitilis in which the aveC gene has been inactivated, or
mutated so as to change the ratio or amount of class 2:1 avermectins
produced.