Co-orthologues of Ribosome Biogenesis Factors in A. Thaliana Are Differentially Regulated by Transcription Factors

Different genes coding for one ribosome biogenesis factor are differentially expressed and are likely under the control of distinct transcription factors, which contributes to the regulatory space for ribosome maturation. Maturation of ribosomes including rRNA processing and modification, rRNA folding and ribosome protein association requires the function of many ribosome biogenesis factors (RBFs). Recent studies document plant-specific variations of the generally conserved process of ribosome biogenesis. For instance, distinct rRNA maturation pathways and intermediates have been identified, the existence of plant specific RBFs has been proposed and several RBFs are encoded by multiple genes. The latter in combination with the discussed ribosome heterogeneity points to a possible function of the different proteins representing one RBF in diversification of ribosomal compositions. Such factor-based regulation would require a differential regulation of their expression, may be even controlled by different transcription factors. We analyzed the expression profiles of genes coding for putative RBFs and transcription factors. Most of the genes coding for RBFs are expressed in a comparable manner, while different genes coding for a single RBF are often differentially expressed. Based on a selected set of genes we document a function of the transcription factors AtMYC1, AtMYC2, AtbHLH105 and AtMYB26 on the regulation of different RBFs. Moreover, on the example of the RBFs LSG1 and BRX1, both encoded by two genes, we give a first hint on a differential transcription factor dependence of expression. Consistent with this observation, the phenotypic analysis of RBF mutants suggests a relation between LSG1-1 and BRX1-1 expression and the transcription factor MYC1. In summary, we propose that the multiple genes coding for one RBF are required to enlarge the regulatory space for ribosome biogenesis.