Defining cis-acting elements and trans-acting factors in RNA localization

Research over the last 10 to 15 years has revealed that intracellular RNA localization is a widespread phenomenon found in a large range of different cell types in an equally impressive number of different organisms (Bashirullah et al., 1998; St. Johnston, 1995). Efforts have focused both on the molecular mechanisms involved in localizing RNAs to particular intracellular targets and on the functional importance (to the cell) of placing certain RNAs at particular cellular sites. In many cases, an understanding of the role of RNA localization seems to be predicated on a careful analysis of how a particular RNA achieves its characteristic distribution. A generalized model of RNA localization usually invokes cellular factors recognizing RNA target sequences. This review will focus on several systems in which cis-acting elements and trans-acting factors recognizing these elements are involved in RNA localization: how they have been defined, how they relate to each other, and how they interact and function to help achieve defined intracellular localization. Conservation of both RNA elements and protein factors across species suggests that RNA localization is probably a fundamental cellular process.