In this paper, we describe an RNA transport system in C. albicans that localizes specific mRNAs to daughter cells in budding yeast and the tip cells of hyphae. When this RNA transport is inactivated by elimination of She3 (a component of the transport system), hyphae display specific defects, including aberrant growth and decreased capacity to damage an epithelial cell monolayer.
We identified mRNAs transported by this system through their tight association with She3, and we used FISH to show that the transported mRNAs accumulate in yeast buds and in the tips of hyphae in a She3-dependent manner. We believe that this study represents the first description of a set of mRNAs that are specifically localized to hyphal tip cells of a filamentous fungus.
Based on direct studies in C. albicans or characterization of orthologous genes in S. cerevisiae, the mRNAs bound by C. albicans She3 are predicted to encode several classes of proteins. Several (orf19.3356, MSS4, CDC20, orf19.267, orf19.3071, orf19.5537, CHT2, orf19.6044 and orf19.6705) encode proteins that function in mitosis, the cytoskeleton, cell wall dynamics, or cell polarity. Another group of associated mRNAs (ASH1, CTA9, CTA9, BCR1, HAC1, GLN3) encode transcriptional regulators. She3 also associates with mRNAs for cell-surface proteins, including predicted GPI anchored proteins (PGA55, YWP1, PGA6, and PGA54) and cell membrane-associated regulators of ion transport (orf19.1582 and orf19.5406). Finally, She3-associated RNAs encode known hyphal-specific virulence factors, RBT4 and SAP5. Taken together, the identities of transported mRNAs suggest that the She3 machinery supports diverse functions, including localization of the basic machinery required for cellular growth and polarity, specification of transcriptional programs in daughter cells and in hyphal tip cells, and differential distribution of cell surface and secreted proteins, some of which function in virulence.
We present several lines of evidence that She3-mediated RNA transport, although not required for hyphal formation per se, is required for normal hyphal growth and function. First, she3Δ/she3Δ strains display reduced ability form hyphae and to penetrate solid agar. Second, although she3Δ/she3Δ strains can form hyphal structures in certain conditions, these filaments are morphologically abnormal. Third, a she3Δ/she3Δ strain shows reduced capacity to damage an epithelial cell monolayer. Finally, we constructed and tested deletions of 33 genes whose transcripts are She3-bound. Approximately one third of the individual deletion mutants have filamentation defects on solid medium, and the aberrant morphologies vary considerably among the mutants. As might be expected, none of these strains displayed exactly the same defects as the she3Δ/she3Δ strains, indicating that the she3 mutant phenotype does not reflect the absence of a single transported mRNA in hyphal tip cells. Taken together, these observations support the idea that transport of multiple mRNAs to hyphal tip cells contributes to proper hyphal function.
Our analysis of the She system in C. albicans allows for the first direct cross-species comparison of an RNA transport system. A surprising finding from our studies is the minimal apparent overlap between She-associated transcripts in S. cerevisiae and C. albicans: only two genes (out of 40 in C. albicans and 24 in S. cerevisiae) are bound in both species. These results suggest that specific mRNAs have moved in and out of the She3-dependent transport system relatively rapidly over evolutionary timescales.
Based on existing data, two plausible mechanisms could account for the apparently rapid evolution of mRNAs transported by the She system. In one model, changes in the RNA-binding specificity of the modular She complex could account for this difference. In an alternate model, which we favor, the change in She3 cargo may have arisen via changes in the nucleotide sequences of the transported mRNAs, which have brought new transcripts under She3 regulation. The cis
-acting elements that mediate localization of She-associated transcripts in S. cerevisiae
have been characterized for a small subset of transported RNAs and are composed of short degenerate sequence motifs, as well as secondary structures that are largely sequence-independent 
. It is plausible that, over evolutionary timescales, small sequence changes mediate rapid losses and gains of cargo mRNAs. Such a mechanism is analogous to evolutionary changes in transcription circuitry, where the basic transcriptional machinery and its regulators have been conserved over long timescales, but changes in cis-regulatory sequences have brought new sets of genes in and out of control of ancient regulators 
C. albicans and S. cerevisiae diverged from a common ancestor roughly 200 million years ago, and since that time they have adapted to distinct environmental niches. S. cerevisiae is widely distributed in the environment, whereas C. albicans is restricted to warm-blooded animals. We suggest that the differences in the She3-transported mRNA cargos likely reflect the distinct pressures of each organism's environmental niche.