Environmental stresses inhibit splicing in the aquatic fungus Blastocladiella emersonii

During nucleotide sequencing of cDNA libraries constructed using RNA isolated from B. emersonii cells submitted to heat shock and cadmium stress, a large number of ESTs with retained introns was observed. Among the 6,350 ESTs obtained through sequencing of stress cDNA libraries, 181 ESTs presented putative introns (2.9%), while sequencing of cDNA libraries from unstressed B. emersonii cells revealed only 0.2% of ESTs containing introns. These data indicate an enrichment of ESTs with introns in B. emersonii stress cDNA libraries. Among the 85 genes corresponding to the ESTs that retained introns, 19 showed more than one intron and three showed three introns, with intron length ranging from 55 to 333 nucleotides. Canonical splicing junctions were observed in most of these introns, junction sequences being very similar to those found in introns from genes previously characterized in B. emersonii, suggesting that inhibition of splicing during stress is apparently a random process. Confirming our observations, analyses of gpx3 and hsp70 mRNAs by Northern blot and S1 protection assays revealed a strong inhibition of intron splicing in cells submitted to cadmium stress.In conclusion, data indicate that environmental stresses, particularly cadmium treatment, inhibit intron processing in B. emersonii, revealing a new adaptive response to cellular exposure to this heavy metal.Exposure to environmental stresses leads to the disruption of many intracellular processes, in particular those carried out by macromolecular complexes, which are extremely sensitive to perturbation by stress conditions [1]. An example of a macromolecular complex that could be affected by environmental stresses is the spliceosome, which is responsible for intron excision, an important cellular process. The spliceosome is a multicomponent complex formed by hundreds of proteins and five small nuclear RNAs (U1, U2, U4, U5 and U6 snRNAs) assembled on the newly synthesized precursor messenger RNA (pre-mRNA) [