The signal recognition particle (SRP) is a ubiquitous ribonucleoprotein (RNP) that mediates targeting of presecretory proteins to the eukaryotic endoplasmic reticulum (ER) or bacterial plasma membrane. The RNA component of the Saccharomyces cerevisiae SRP (termed scRI) is ~200 nucleotides longer than its mammalian counterpart. Eukaryotic SRP comprises two domains, termed Alu- and S-. Most of the 'extra' material is in the Aludomain as helical insertion elements. Five of six SRP proteins are well conserved from yeast to mammals, but the yeast SRP9 homologue, Srp21 p, has a unique C-terminal region. that comprises more than 50% of its length. In this thesis, interactions and functions of Srp21 p and Srp14p, the two yeast Alu-domain proteins, are analysed. Srp14p co-purifies with Srp21 p when they are co-expressed in E. coli. This interaction is salt sensitive, being lost at higher concentrations (> 250 mM). Purified Srp21 p was found to bind scR I independently as well as in concert with Srpl4p in vitro. However, whilst Srpl4p binds a minimal Alu-domain RNA, Srp2lp only binds RNAs containing the Alu-domain and the majority ofthe S-domain. Much of the C-terminal domain of Srp21 p is not necessary for its function. However, progressively larger deletions result in slower growth, and loss of SRP function. Genetic interactions were observed between C-terminally truncated Srp21 p and scR I mutants in both the Alu- and S-domain. These results are consistent with in vitro binding data and suggest that Srp21 p interacts with elements in both domains of SRP. Computational and biophysical analysis indicated that the Cterminus of Srp2lp may be highly unstructured. Hydroxyl-radical footprinting provided evidence for both Srp14p and Srp21 p binding the Alu-domain of scR I, Srpl4p to the conserved core and Srp2lp to a yeast specific helix.