Tuesday's blast was the latest in a
spate of violence in Istanbul
The remote-controlled device exploded as a bus was carrying prosecutors and staff to a state security court (DGM) in the city.
The driver of the bus, a prosecutor, two police officers and a passenger were slightly wounded. Several nearby cars were also damaged.
Istanbul Governor Muammer Guler accused Kurdish or leftist fighters of detonating the bomb.
The attack came two weeks after a bomb a woman was carrying exploded killing her.
Police say she was a suspected member of an underground left-wing group.
Last week there were also two separate blasts near British HSBC bank branches in Istanbul, leaving two people injured.
The latest violence coincides with Turkish press reports of a rift between the country's army and the government that the country's Prime Minister Tayyip Erdogan has been anxious to quell.
The powerful Turkish military is worried that Ankara is giving jobs to prominent Islamists.
The army considers itself the protector of modern Turkey's secular principles while the main party in Parliament, the Justice and Development Party, has its roots in now-disbanded Islamist parties.