(There were some exceptions of course throughout Greek history with the appointments of Tsimas, Papanghelopoulos and Dravillas).
Roubatis was born in 1948 in Ioannina and studied in the USA. He is said to hold a PhD in International Relations from the Johns Hopkins University.
He served as the Washington correspondent for the socialist To Vima newspaper for quite a few years and after the rise of Andreas Papandreou in 1981, Roubatis became a special advisor to the then PASOK PM on international issues.
In March 1987 he assumed a role in Papandreou's government and remained in this position until June 1988 when Andreas Papandreou replaced him with Dimitri Maroudas.
It should also be noted that he served as PASOK's eurodeputy from 1994-1999 and was an extremely close associate to the founder of PASOK Andreas Papandreou.
(As we can clearly see, more and more hardline PASOK veterans are assuming more and more posts in the government.)
Sources - enikos, protothema, parapolitika