Post-transcriptional control of sucrose synthase expression in anaerobic seedlings of maize

We have examined post-transcriptional control of expression of the anaerobically induced sucrose synthase 1 (SS1) isozyme mRNA encoded by the shrunken (Sh) gene of maize (Zea mays L.). The SS1 transcript level is increased in maize seedling roots during anaerobiosis without a concomitant increase in the SS1 protein level. We show that the anaerobic SS1 RNA was loaded onto polyribosomes and that SS1 proteins produced by in vitro translation of polyribosomal RNA from anaerobic roots and immature kernels were indistinguishable based on abundance and apparent molecular weight. [(35)S]Methionine uptake in control and anaerobically stressed seedling roots indicated a detectable, but only slight, increase in radiolabel in the SS1 polypeptide as compared to the sucrose synthase 2 isozyme, SS2. However, this slight increase in [(35)S]methionine uptake did not contribute to a detectable increase in the steady state level of SS1 protein relative to SS2 protein. Chase experiments with unlabeled methionine indicated that SS1 protein was relatively stable in the anaerobic environment. From these results we conclude that SS1 protein was not rapidly turned over in the anaerobic environment and that expression of anaerobically induced SS1 transcripts was blocked at some step beyond polyribosomal loading.