A-factor is a protein which is found in the bacterial genus Streptomyces. This protein helps start the production of the antibiotic streptomycin and the process of morphological differentiation in biochemical reactions. It is used in biotechnology to induce these reactions in the cells of mutant strains of Streptomyces that can't produce it naturally.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.