Ribosomal protein genes are among the most highly expressed genes in most cell types. Their products are generally essential for ribosome synthesis, which is the cornerstone for cell growth and proliferation. Many cellular resources are dedicated to producing ribosomal proteins and thus this process needs to be regulated in ways that carefully balance the supply of nascent ribosomal proteins with the demand for new ribosomes. Ribosomal protein genes have classically been viewed as a uniform interconnected regulon regulated in eukaryotic cells by target of rapamycin and protein kinase A pathway in response to changes in growth conditions and/or cellular status. However, recent literature depicts a more complex picture in which the amount of ribosomal proteins produced varies between genes in response to two overlapping regulatory circuits. The first includes the classical general ribosome‐producing program and the second is a gene‐specific feature responsible for fine‐tuning the amount of ribosomal proteins produced from each individual ribosomal gene. Unlike the general pathway that is mainly controlled at the level of transcription and translation, this specific regulation of ribosomal protein genes is largely achieved through changes in pre‐mRNA splicing efficiency and mRNA stability. By combining general and specific regulation, the cell can coordinate ribosome production, while allowing functional specialization and diversity. Here we review the many ways ribosomal protein genes are regulated, with special focus on the emerging role of posttranscriptional regulatory events in fine‐tuning the expression of ribosomal protein genes and its role in controlling the potential variation in ribosome functions. This article is categorized under: Translation > Ribosome Biogenesis Translation > Ribosome Structure/Function Translation > Translation Regulation
The plastid psbB operon is composed of the psbB, psbT, psbH, petB and petD genes. The psbN gene is located in the intergenic region between psbT and psbH on the opposite DNA strand. Transcription of psbN is under control of sigma factor 3 (SIG3) and psbN read-through transcription produces antisense RNA to psbT mRNA. To investigate on the question of whether psbT gene expression might be regulated by antisense RNA, we have characterized psbT sense and antisense RNAs. Mapping of 5′ and 3′-ends by circular RT–PCR and /or 5′-RACE experiments reveal the existence of two different sense and antisense RNAs each, one limited to psbT RNA and a larger one that covers, in addition, part of the psbB coding region. Sense and antisense RNAs seem to form double-stranded RNA/RNA hybrids as indicated by nuclease digestion experiments followed by RT–PCR amplification to reveal nuclease resistant RNA. Western immunoblotting using antibodies made against PSBT protein and primer extension analysis of different plastid mRNA species and psbT antisense RNA suggest that sequestering of psbT mRNA by hybrid formation results in translational inactivation of the psbT mRNA and provides protection against nucleolytic degradation of mRNA during photooxydative stress conditions.
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