Two cDNAs encode two nearly identical Cu/Zn superoxide dismutase proteins in maize

Area of Science:

• Plant Molecular Biology
• Enzymology
• Genetics

Background:

• Superoxide dismutase (SOD) enzymes are crucial for cellular defense against oxidative stress.
• Maize cytosolic SOD-4 was initially characterized as a single protein species.
• Purification and N-terminal sequencing revealed ambiguity at residue 12.

Purpose of the Study:

• To investigate the molecular basis for the observed heterogeneity in maize SOD-4 protein.
• To identify and characterize the genes encoding maize SOD-4.
• To confirm the expression of these genes at the RNA level.

Main Methods:

• Purification of maize SOD-4 protein and N-terminal amino acid sequencing.
• Construction and screening of a maize lambda gt11 cDNA library.
• DNA sequencing, restriction mapping, RNA, and genomic DNA blot analyses.

Main Results:

• Two distinct cDNAs encoding SOD-4 were isolated and sequenced.
• Both cDNAs perfectly matched the N-terminal sequence of the purified protein, except at codon 12.
• One cDNA coded for glutamic acid, the other for aspartic acid at position 12, explaining the protein heterogeneity; RNA and DNA blots confirmed two distinct genes.

Conclusions:

• Two maize genes encode indistinguishable SOD-4 proteins, differing only by a single amino acid substitution (Glu/Asp) at position 12.
• This genetic variation leads to the observed heterogeneity in the purified SOD-4 protein.
• The study confirms the existence and expression of two distinct genes for maize SOD-4.