Chapter 20 Regulation of Gene Expression in Eukaryotes

1. It was once believed that eukaryotes do not have operons, but recent discoveries in nematodes indicate otherwise. Caenorhabditis elegans contains operons as well as typical eukaryotic genes with introns.
2. In nematodes, the operons are controlled from a single promoter, as in prokaryotes.
a. Unlike prokaryotes, however, only one protein can be produced from the mRNA.
i. Ribosomes cannot reinitiate at a different start codon on the eukaryotic mRNA.
ii. Instead, pre-mRNAs are processed into monogenic mRNAs for individual translation.
b. Processing of pre-mRNAs is shown in Figure 20.1:
i. RNA polymerase II produces a capped polygenic pre-mRNA.
ii. Cotranslational processing includes transsplicing and generation of the 3¢ end by cleavage and polyadenylation.
iii. Transsplicing using snRNP puts SL-RNA (splice leader) onto the 5¢ end of a gene in the operon, making the donated SL‑RNA the leader sequence for each mRNA in the operon.
iv. Cleavage and polyadenylation generate 3¢ ends.
c. About 15 percent of C. elegans genes are in operons that range from 2–8 genes in size.
i. Unlike in prokaryotes, no single operon includes all proteins needed for a pathway or a multiprotein complex.
ii. Often, genes that work together will be in an operon with genes of unrelated function.