The Clamshell Shape of IstB (IMAGE) Johns Hopkins Medicine Caption Five of IstB's 10 subunits are in the foreground with arrows pointing to the ends of a segment of DNA, bent 180 degrees by the clamshell-shaped protein. Credit Cell Press Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.