Argenine - If you go to the old thread linked above, you'll find a link to this site: Porgy And Bess, detailing the development of it. Briefly, DuBose wrote a novel, he and his wife Dorothy dramatised it for stage in 1927, and he and the Gershwins developed Porgy And Bess from it. As far as I know DuBose alone was responsible for the libretto for Porgy And Bess.

According to Brahms And Sherrin, which I cite above, the songwriting contributions were:

  DuBose Alone
    Summertime
    My Man's Gone Now
    A Woman Is A Sometime Thing
    The Buzzard Song
  
  DuBose writes, Ira edits to make singable
    Bess, You Is My Woman
  
  DuBose And Ira, full collaboration
    I Got Plenty Of Nuttin'
    It Takes A Long Pull To Get There
  
  Ira Along
    There's A Boat Dat's Leavin' Soon For New York
    It Ain't Neccessarily So
  

  Mick