Credit: FOODPIX

For people who must restrict their protein intake — such as patients with kidney failure — a mutant rice that is naturally low in proteins called glutelins is beginning to be used as a dietary therapy. Makoto Kusaba and colleagues have now discovered how this mutant achieves low glutelin levels (Plant Cell doi:10.1105/tpc.011452; 2003). The answer involves the increasingly well-known biological phenomenon of RNA interference.

Glutelins are the major proteins in cereal grains such as wheat and rice. They are produced from two families of genes, the GluA and GluB families, which occur on at least three different chromosomes. Several mutations in rice disrupt one or another of these genes, but do not significantly reduce the total amount of glutelin produced. But the mutation studied by Kusaba et al. — 'low glutelin content-1', or LGC-1 — has a much broader effect, completely abolishing production of one GluB protein, radically reducing the levels of other GluBs, and even limiting the GluA content.

Kusaba et al. have now found that LGC-1 plants lack a large stretch of DNA between two of the GluB genes, GluB4 and GluB5. This region may encode a very short protein of about 25 amino acids, but loss of this protein does not appear to be the cause of the low glutelin levels. Crucially, the deletion also removes the 'stop' signal from the end of GluB5, so that the messenger RNA produced from this gene runs on into the GluB4 mRNA. As it happens, these two genes have opposite orientations on the chromosome, and their sequences are almost completely complementary, so the mRNA folds over and the two sequences can bind to each other, forming double-stranded RNA.

Double-stranded RNA is involved in sequence-specific suppression of gene expression in organisms from plants to fungi to animals. The first step in this RNA-interference process is the production of smaller fragments, called small inhibitory RNAs, from a double-stranded RNA. These fragments act as templates to guide suppression of specific genes by mechanisms including methylation. Kusaba et al. found potential small inhibitory RNAs, as well as increased methylation of glutelin genes, in LGC-1 plants. The similarity between glutelin gene sequences means that suppression is directed towards all GluB genes, and even spills over onto GluA genes.

RNA interference is fast becoming an invaluable, if sometimes unpredictable, tool in the molecular biologist's armoury. It has now found yet another use. The promiscuous gene silencer that happenstance has produced in LGC-1 plants might, when inserted as multiple copies into otherwise normal rice, result in still lower glutelin levels — a 'super-low-protein' rice.