R. Freund, M. Oswald:
"Tissue P Systems with Symport/Antiport Rules of One Symbol are Computationally Universal";
in: "Cellular Computing (Complexity Aspects)", M.A. Gutiérrez-Naranjo, Gh. Paun, M. Pérez-Jiménez (ed.); Fénix Editora, Sevilla, 2005, 187 - 200.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936351