5:15 PM PT -- The State of Missouri followed through with the execution by lethal injection, and Marcellus Williams was pronounced dead at 6:10 PM CT.

Busta Rhymes is going to bat for a Missouri man who's scheduled to be executed later today -- but hip hop's Dungeon Dragon insists the state's ignoring evidence as time runs out.

TMZ Hip Hop obtained video of Busta's PSA on behalf of Marcellus Williams, the inmate whose life is at the mercy of Gov. Mike Parson ... who could still issue a stay of execution. However, he recently rejected the inmate's clemency request for life in prison.

Busta passionately argues the DNA evidence prosecutors later discovered should be more than enough to vacate Williams' execution, and facts -- not politics -- should guide Gov. Parson to seek justice.

Williams was convicted of the 1998 murder of a St. Louis woman named Felicia Gayle, who was stabbed to death during a home invasion burglary in her home, but he's maintained his innocence.

The Midwest Innocence Project had sought to get his conviction overturned based on not only the DNA, but also the fact prosecutors excluded a juror based on his race. The state Supreme Court had already denied that appeal -- and, on Tuesday, the U.S. Supreme Court denied a petition to stay his execution.