Alternate Splicing of a Plant mRNA [Werneke et al., The Plant Cell 1:815 (1989)]The plant example of alternate is for the gene encoding the enzyme RUBISCO activate. RUBISCO activase is a soluble, nuclear-encoded chloroplast enzyme. This abundant protein, is about 2% of the soluble leaf protein in spinach. Two molecular forms of the protein are recognized. In spinach, the proteins are 41 kd and 45 kd polypedtides, and the proteins are 44 kd and 47 kd polypeptides in Arabidopsis. The monocot corn is different and has only one protein. This enzyme is required to activate the enzymatic activity of RUBISCO, the first step in the conversion of the electromagnetic energy collected during the light reaction of photosynthesis in chemical energy in the forms of sugars.
The RUBISCO activase gene has been cloned. cDNA clones had been isolated previously and revealed two nucleotide binding sites. When the cDNA was expressed in E. coli only one polypeptide was detected. Antibodies from each of the 41 and 45 kd strongly crossed reacted with each other suggesting a high degree of homology between the proteins. Furthermore genomic southerns with the cDNAs showed that both spinach and Arabidopis had a single gene. How can this be explained ?
Additional cDNA clones were isolated and sequenced for both spinach and Arabadopsis. The following table describes the differences between the two types of cDNAs.
Genomic clones were then isolated for each of the two species and sequenced. The sequence data revealed that each of the genes contained seven exons and six introns. Comparison of the sequences of the genomic and cDNA clones of spinach revealed the following manner in which the two different types of cDNAs could be generated. The full length mRNA that gave rise to the larger protein was generated by splicing at the terminal-most 5' and 3' splicing sites. 22-bp into intron 6 is another 5' splice junction sequence. Alternate splicing occurs at this site to generate the mRNAs that are translated into the shorter proteins. This 22-bp sequence can be considered an extension of exon 6 and is called an auxon for alternative exon. Alternative splicing events can form different mRNAs from a single primary transcript. Thus an exon can actually be represented as an intron in another transcript.
Copyright © 1998. Phillip McClean