Fig. 7. Expression of the gapA and rtoA genes in parental and Dd-STATc null cells. Ax2 cells and a Dd-STATc null cells were subjected to development and induced with either sorbitol or DIF, as described in the legend to Fig. 1. After 15 minutes of induction total cellular RNA was isolated and analysed by northern transfer. The blot was hybridised sequentially with the indicated probes. Ig7 is a constitutively expressed RNA that serves as a loading control. The gapA mRNA migrates as a broad band at the expected size of 3.2kb (Adachi et al., 1997). The rtoA transcript is reported to be 1.6 kb and we also observed a band of 1.6kb when cells were growing or developing on a substratum (data not shown). However, when cells were developing in suspension we again observed a band of 1.6 kb, but there is an additional, higher molecular weight species of 2 kb. Because the two RNAs show parallel concentration changes we believe that the longer species is an RNA processing variant of the shorter species. There is a 430 nt intron in the rtoA gene and it has a very unorthodox (GG) splice donor site (Wood et al., 1996). The size difference between the longer and shorter transcripts (c. 2 kb minus c. 1.6 kb=0.4 kb) is therefore consistent with a splicing defect in the cells placed in suspension. The result shown is for a Dd-STATc null strain generated using a construct containing a blasticidin casette. We also analysed strains generated using a hygromycin disruption casette and, in this case, we compared homologous integrants (i.e. Dd-STATc disruptants) and non-homologous, random integrants from the same transformation (data not shown). This strategy corrects for any unsuspected genetic divergence between the parental strain and the null strain. Again, expression of gapA and rtoA in the Dd-STATc disruptants was unresponsive to osmotic stress (data not shown).