LLM-infused tools have entered the newsroom, transforming journalistic work practices. A few studies have investigated how LLMs influence journalistic practices, but there is a lack of research on how to design LLM-infused tools to support trustworthy journalism. In this paper, we explore how prototyping within a defined design space can help identify and explore the challenges of trustworthy LLM integration in journalism. We conduct eight interviews with news industry stakeholders and identify five challenges for trustworthy journalism arising with the introduction of LLMs: factuality, neutrality, autonomy, efficiency, and AI literacy. Based on these challenges, we map a design space and iteratively explore four prototypes of interactive interfaces promoting trustworthy LLM-infused journalism, which are evaluated with news industry stakeholders. We discuss opportunities and conflicts within the design space, how interactive interfaces can be used to concretize guidelines for AI use, and challenges in incorporating Explainable AI into everyday tools for journalists.