Nucleon is an incredibly powerful energy source capable of being used as fuel by Transformers and other mechanoid life forms, but though it can give great strength and power and even revived the nonfunctional, it often has terrible side effects. Nucleon is said to affect no two mechanoids the same. One very common effect for Transformers is the loss of transformation ability. The Action Masters resulted from injudicious use of this mysterious, dangerous substance.
Nucleon appears as a funky liquid with a dangerous glow. It looks like fun, and seems to help, but don't be a fool! Listen to your momma and stay off the stuff!
Toy packaging blurb
According to the toy packaging Optimus Prime went searching for new energy sources and, within the heart of a black hole discovered Nucleon: a miracle fuel that made Transformers stronger, faster, and more alive, but cost them the ability to transform. Soon the Decepticons stole the technology and created Action Masters of their own.
This story was later referenced with the Japanese release of Nucleon Quest Convoy, a black repaint of Powermaster Optimus Prime who was given the backstory of being Prime specially outfitted for this trip into the black hole.
Marvel Comics continuity
Grimlock heard rumors of a new miracle fuel on the planet Hydrus Four. Against orders, he stole the non-functioning bodies of his fellow Dinobots and headed there to find out. What he descovered was a ruined world filled with creepy, obsessive, rotting zombie-like beings warning him that it was all because of Nucleon. Being Grimlock, he then went ahead and used the fuel anyway, first on himself then to revive his fallen teammates.
Grimlock soon found parts of his body inexplicably seizing up, and began to fear that he should have listened to the creepy dying robots after all. After returning to the Ark, Grimlock used the Nucleon to repair all the damaged Autobots in the ship's medical bay (as well as Megatron, whom he didn't know was there). Following the Transformers battle with Unicron, Grimlock froze-up entirely. It was then that Hi-Q (in the process of merging with Optimus Prime) saw with "Powermaster eyes" that Grimlock wasn't malfunctioning, but changing. He used his Powermaster abilities to speed-up the process and Grimlock burst forth in his new, more-powerful Action Master form.
Meanwhile, Hi-Q's own mental merger with Prime was also accelerated by his contact with Nucleon. Soon afterward, the Last Autobot re-created Hi-Q as Optimus Prime, looking very much like his Action Master toy and never being seen to transform. However, while Grimlock explicitly said that his own transformational abilities had been lost, there was no such statement for Prime. So while the involvement of Nucleon and Prime's general appearance suggest that he had become an Action Master too, the uniqueness of his re-creation makes things less certain.
As in other fiction, neither Grimlock nor Prime kept their Action Master bodies for long. However, Snarl is seen as an Action Master (and his partner Tyrannitron even gets a mention). Given Snarl's hatred of his own dinosaur mode, his remaining an Action Master was likely a matter of taste.
Swoop however was faced with another situation as shortly after being revived with Nucleon, he was caught in an explosion at a Nucleon processing factory and even more severely mutated. The Autobot Mini-Cons stepped in and were able to get that reaction under control, but Swoop himself was transformed into a (very powerful) Mini-Con by the Nucleon.
According to Scattorshot, nucleon is a controlled substance in Axiom Nexus. Cheetor was unconcerned when it was pointed out that Jackpot had this contrabrand, as Jackpot did not get the Nucleon in this universe.