Bailed-out US banking giant Citigroup last night brought in a mechanism to deter investors from owning over 5 per cent of its stock. It is designed to protect a $43bn (£26bn) tax break it receives if under 50 per cent of its shareholders own over 5 per cent. The move came as it launched its anticipated $58bn (£35.4bn) conversion of preferred shares into common stock, leaving the US government with a 34 per cent stake. <br />