Post-Transcriptional Regulation of Catalase Isozyme Expression in Cotton Seeds

We reported previously that expression of the five tetrameric catalase isozymes during postgerminative growth of cotton seedings was a consequence of interactions between two subunits (SU 1 and SU 2) temporally synthesized from two distinct catalase genes. In this study, we focused on the regulation of the expression of these two catalase subunits during the changeover from glyoxysomal to leaf-type peroxisomal metabolism. The steady-state level of glyoxysomal SU 1 protein (present in 12-hour-old seeds) increased through day 3 and then declined linearly through day 6, whereas SU 2 protein (first detected in 24-hour-old seeds) increased continuously through day 6. The time courses for steady-state levels of the mRNAs encoding these two subunits revealed two clearly separated peaks: the first at day 1 (SU 1) and the other at day 4 (SU 2). Accumulation of these mRNAs preceded the accumulation of their corresponding proteins by at least 24 hours, suggesting temporal, pretranslational regulation of synthesis of both subunits. Results from run-on transcriptional assays with isolated nuclei, however, revealed that transcripts encoding both subunits were synthesized together on days 1 through 5. Hence, it appears that the accumulations of SU 1 and SU 2 mRNAs are controlled primarily at the post-transcriptional level, which has not been reported for catalase or any other eukaryotic peroxisomal enzymes. The accumulation of SU 1 mRNA is not light dependent, whereas the accumulation of SU 2 mRNA, which directs synthesis of the predominant subunit comprising the leaf-type peroxisomal isozyme, occurs only after exposure of seedlings to light.