The Organization, Function, and Evolution of Gene Clusters in Eucaryotes

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Abstract
Present genetic and biochemical evidence suggests that 2 basic types of gene clusters occur in eukaryotes. The 1st type encodes an enzyme aggregate (multienzyme complex). In an increasing number of instances, such gene clusters are cluster-genes, i.e., single genes encoding multifunctional polypeptide chains, typically associated as homopolymeric aggregates. In several instances, a channeling role for such enzyme aggregates is supported, i.e., the aggregate serves to sequester intermediates in a particular biochemical pathway and to prevent competition between 2 potential competitive pathways. Gene clusters are considered to have evolved by retroevolution involving successive tandem gene duplications followed by functional divergence. Apparently, natural selection often resulted in the further evolution of such gene clusters into cluster-genes by the process of gene fusion, especially in eukaryotes. The 2nd (and apparently less frequent) type of gene cluster in eukaryotes is a true cluster of contiguous but separate genes which do not encode an enzyme aggregate. Certain of these cluster appear to have operon-like characteristics. In the absence of evidence for polycistronic mRNA production, at least 1 of these clusters (the qa cluster in Neurosporas sp.) appears to be maintained by natural selection because it contains a gene encoding a regulatory protein which is primarily cis acting.