Non-coordinate regulation of 5S rRNA genes and the gene encoding the 5S rRNA-binding ribosomal protein homolog in Neurospora crassa

In eukaryotes, the levels of ribosomal proteins are coordinately regulated under varying nutritional conditions and at different developmental stages. Little is known about how ribosomal protein levels are coupled to the levels of rRNA. The formation of a ribonucleoprotein particle composed of 5S rRNA and a ribosomal protein is an early step in ribosome assembly. To investigate how these two ribosomal components are regulated in Neurospora crassa, we cloned the gene encoding the 5S rRNA-binding ribosomal protein (crp-4) and developed a novel system for measuring relative 5S rRNA transcriptional rates in vivo, using a reporter RNA derived from the 40S precursor RNA. The reporter RNA is cleaved from the 5S rRNA in vivo and therefore allows us to distinguish between changes in the 5S rRNA transcription rate and 5S rRNA stability. Using this system, we found that transcription of 5S rRNA is constitutive and is not coordinated with the levels of crp-4 mRNA or with 40S rRNA levels during a carbon upshift or a carbon downshift.