SEPARATE MOLECULAR MECHANISMS REGULATE CAT2 GENE-EXPRESSION BEFORE AND AFTER GLYOXYSOME MATURATION IN 2 GENETIC LINES OF MAIZE

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 Cat‐2 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. Early Cat2 gene expression is likely controlled by the compartmentalization of CAT‐2 in glyoxysomes. Copyright © 1990 Wiley‐Liss, Inc.