A role for RNA-binding proteins implicated in pathogenic development of Ustilago maydis

Successful infection by the corn pathogen Ustilago maydis is accompanied by a number of morphological transitions that resemble simple developmental programs. Prerequisite for plant penetration is the formation of an infectious filament that exhibits polar tip growth and forms empty sections at its distal pole. In order to investigate the impact of RNA-binding proteins on such developmental processes, members of this protein class were identified according to sequence similarities with well characterized RNA-binding domains. Out of 94, 25 candidates were chosen and respective gene deletion strains were constructed and tested for pathogenic development. The loss of Rrm4, a protein with three N-terminal RNA recognition motifs (RRM) and a C-terminal protein interaction domain, resulted in reduced filamentation and virulence. Further analysis revealed that deletion strains form shorter, bipolar growing filaments. Subcellular localisation in vivo showed that Rrm4 forms particles that move bi-directionally along microtubules. Further mutational analysis of the various protein domains revealed that the N-terminal RNA contact regions RRM1 and RRM2 are most likely necessary to contact cargo whereas the C-terminal protein interaction domain is crucial for particle formation. These results indicate that Rrm4-containing particles transport RNA from the nucleus to the cell poles and that this process is important for unipolar tip growth of the infectious hyphae. A role for long distance transport of RNA along microtubules has already been implicated for embryo development and neuronal signal transmission. Our results obtained in U. maydis constitute the first example for long distance RNA transport in microorganisms indicating that the basic concept is evolutionarily more conserved than previously anticipated.