Around a fifth of genes in Caenorhabditis elegans reside in operons, so this is an ideal system in which to find distinctions between operon and non-operon genes. An initial survey showed that operon genes are more likely to be required for growth (88% of operon genes are growth genes) and that they are expressed at twice the level of non-operon genes at all life stages. What is even more striking is that operon genes are strongly upregulated at life stages in which the worm exits an arrested stage of development and enters a period of rapid growth: at hatching and after larval stage 1 (L1) or L2 (dauer) arrest. At all of these stages, the expression of non-operon genes stays constant or decreases.
This anticorrelated behaviour of the two gene sets made the authors suspect that operon and non-operon genes compete for transcriptional resources. Transcriptional resources are low during arrest phases, although transcription ramps up rapidly when exiting these stages. The organization of genes into operons might therefore allow cells to make the most of the transcriptional machinery by concentrating it on relatively few promoters.
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