A novel heat stress-responsive gene in the marine diatom Chaetoceros compressum encoding two types of transcripts, a trypsin-like protease and its related protein, by alternative RNA splicing

We investigated the change of mRNA expression patterns in the laboratory-gr own diatom Chaetoceros compressum under heat-stress conditions by mRNA arbi trarily primed (RAP) RT-PCR. Cells grown at 20 degreesC were subjected to h eat treatment at 30 degreesC for 15 min and subsequently maintained at 20 d egreesC for 8 h. Four genes including HI-5 were detected as heat stress-res ponsive genes by fingerprint analysis of RAP RT-PCR. Cloning for full-lengt h cDNA sequences of HI-5 transcripts and related genomic DNA analysis revea led that two types of mRNA, HI-5a and HI-5b, were transcribed from the sing le HI-5 gene. While the HI-5a protein contained a catalytic domain characte ristic to trypsin-like proteases, the HI-5b protein lacked this domain due to an insertion in the associated mRNA of 112 nucleotides; this insertion s equence contained a stop codon near the central region. Quantitative RT-PCR was performed to investigate the changes in expression levels of the two t ypes of mRNA following heat treatment. The HI-5b transcripts were constitut ively expressed in both unstressed and heat-stressed cells. In contrast, th e number of HI-5a transcripts markedly increased in cells immediately after heat stress, reaching levels 19-fold higher at 8 h after heat stress than that in unstressed cells. These results suggest that RNA splicing plays a k ey role in heat stress-dependent expression of the HI-5a and HI-5b transcri pts from the single HI-5 gene in the diatom.