Separate molecular mechanisms regulate CAT2 gene expression before and after glyoxysome maturation in two genetic lines of maize

Abstract

The temporal expression pattern of the CAT-2 catalase isozyme in scutella of Zea mays seedlings normally coincides with that of other major glyoxysomal enzymes. In standard genetic lines (e.g., W64A), the CAT-2 enzyme is synthesized de novo after imbibition, reaches a peak at approximately 4 days later, and then declines steadily. In a high CAT-2 genetic line, R6-67, the enzyme accumulates in a linear fashion for at least 8 days after imbibition and reaches a level 3-fold higher than in W64A. During the first 9 days of early seedling growth in W64A, the correlation between Cat2 mRNA levels and CAT-2 protein suggests that pretranslational control governs Cat2 gene expression. In R6-67, the steady rise in CAT-2 protein appears to result from a pretranslational control mechanism in which Cat2 mRNA apparently never declines to levels which would limit the rate of accumulation of CAT-2 protein. In addition, the amount of Cat2 mRNA bound to polysomes is 3-fold higher in R6-67 at day 9, relative to W64A at day 9, reflecting a much greater capacity to synthesize CAT-2 later in development. Despite substantial differences in Cat2 mRNA levels between genetic lines, early CAT-2 protein accumulation is similar until day 5, when other glyoxysomal enzymes also attain maximal activity levels. The early increase in CAT-2, between day 2 and day 5 post-imbibition, occurs despite a sharp decline in polysomal Cat2 mRNA. This is related to a transient decline in total extractable polysomes which paradoxically coincides with the peak in glyoxysomal enzyme activities.(ABSTRACT TRUNCATED AT 250 WORDS)