SEVERE GROWTH DEFECT IN A SCHIZOSACCHAROMYCES-POMBE MUTANT DEFECTIVE IN INTRON LARIAT DEGRADATION

The cDNAs and genes encoding the intron lariat-debranching enzyme were isolated from the nematode Caenorhabditis elegans and the fission yea st Schizosaccharomyces pombe based on their homology with the Saccharo myces cerevisiae gene. The cDNAs were shown to be functional in an int erspecific complementation experiment; they can complement an S. cerev isiae dbr1 null mutant. About 2.5% of budding yeast S. cerevisiae gene s have introns, and the accumulation of excised introns in a dbr1 null mutant has little effect on cell growth. In contrast, many S. pombe g enes contain introns, and often multiple introns per gene, so that S. pombe is estimated to contain similar to 40 times as many introns as S . cerevisiae. The S. pombe dbr1 gene was disrupted and shown to be non essential. Like the S. cerevisiae mutant, the S. pombe null mutant acc umulated introns to high levels, indicating that intron lariat debranc hing represents a rate-limiting step in intron degradation in both spe cies. Unlike the S. cerevisiae mutant, the S. pombe dbr1::leu1(+) muta nt had a severe growth defect and exhibited an aberrant elongated cell shape in addition to an intron accumulation phenotype. The growth def ect of the S. pombe dbr1::leu1(+) strain suggests that debranching act ivity is critical for efficient intron RNA degradation and that blocki ng this pathway interferes with cell growth.