Though phosphorylation appears to be the main activator for AMPK, some studies suggest that AMP is an allosteric regulator as well as a direct agonist for AMPK. Furthermore, other studies suggest that the high ratio of AMP:ATP levels in cells, rather than just AMP, activate AMPK. For example, the species of "Caenorhabditis elegans" and "Drosophila melanogaster" and their AMP-activated kinases were found to have been activated by AMP, while species of yeast and plant kinases were not allosterically activated by AMP.