Synthetic biology, a converging science and technology characterized by the adoption of an engineering approach to life, shows several applications. However, it could also generate numerous risks. This work aims at individuating a regulatory framework and a model of governance for addressing the risks arising within synthetic biology area. This is to ensure that the progress is not hindered but, at the same time, the problematic issues are not neglected. The suggested model is named "prudent vigilance" (inspired by the report about synthetic biology, drafted by the U.S. Presidential Commission on Bioethics, 2010). It entails: (a) an ongoing and periodically revised assessment and management of the risks, assembling all the stakeholders in a democratic way, and (b) the adoption of policies, from the "top down" and "bottom up" levels, that are based on the principle of proportionality (among benefits and risks), on a reasonable balancing between different interests and rights at stake, and on a constitutional frame, i.e. the protection of human rights. In the end, the operability of the model is checked by considering, as a case study, the biosecurity risks of synthetic biology.