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

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Abstract
We investigated the change of mRNA expression patterns in the laboratory‐grown diatom Chaetoceros compressum under heat‐stress conditions by mRNA arbitrarily primed (RAP) RT‐PCR. Cells grown at 20 °C were subjected to heat treatment at 30 °C for 15 min and subsequently maintained at 20 °C for 8 h. Four genes including HI‐5 were detected as heat stress‐responsive genes by fingerprint analysis of RAP RT‐PCR. Cloning for full‐length cDNA sequences of HI‐5 transcripts and related genomic DNA analysis revealed that two types of mRNA, HI‐5a and HI‐5b, were transcribed from the single HI‐5 gene. While the HI‐5a protein contained a catalytic domain characteristic to trypsin‐like proteases, the HI‐5b protein lacked this domain due to an insertion in the associated mRNA of 112 nucleotides; this insertion sequence contained a stop codon near the central region. Quantitative RT‐PCR was performed to investigate the changes in expression levels of the two types of mRNA following heat treatment. The HI‐5b transcripts were constitutively expressed in both unstressed and heat‐stressed cells. In contrast, the 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 key role in heat stress‐dependent expression of the HI‐5a and HI‐5b transcripts from the single HI‐5 gene in the diatom.