The COL1 gene was isolated from Ophiostoma novo-ulmi using the techniques of insertional mutagenesis and plasmid rescue. Sequence analyses suggest that the COL1 gene encodes a unique protein of 826 amino acids with consensus-type RNA-binding domains, most similar to a putative protein from Schizosaccharomyces pombe which resembles the C-terminus of the Saccharomyces cerevisiae U4/U6 splicing factor PRP24. Disruption of the COL1 gene produced the yeast-like col1 mutant. The inability of the mutant to synthesize the COL1 gene product was confirmed by transcript analysis. Transformation of the col1 mutant with the COL1 gene restored the wild phenotype and production of the 4.0-kb mRNA. The results from this study demonstrate that the COL1 RNA-binding protein is associated with filamentous growth in O. novo-ulmi.