This entry represents a group of medium/long-chain-fatty-acid--CoA ligases, including FadD6/FadD17 from Mycobacterium tuberculosis, CasI from Rhodococcus jostii and Fat1 from budding yeasts.Fad6 and FadD17 catalyse the activation of long-chain fatty acids as acyl-coenzyme A (acyl-CoA), which are then transferred to the multifunctional polyketide synthase (PKS) type III for further chain extension [, , , ]. CasI catalyzes the ATP-dependent formation of CoA thioesters of steroids with isopropanoyl side chains, likely occurring as degradation intermediates [, ].Fat1 is a very long-chain fatty acyl-CoA synthetase that may also play a role in transport of long chain fatty acids [].