Expression of ribosomal protein genes during Xenopus development.

The Xenopus ribosomal protein genes provide an excellent system to elucidate the complex regulation encompassing 60 functionally related proteins present in equimolar amounts in ribosomal subunits. Oogenesis and embryogenesis provide unique opportunities to investigate ribosome biosynthesis in situations wherein gene activation of individual components is uncoupled from assembly of the ribosomal subunits. This chapter has focused on the basic parameters that control ribosomal protein gene expression during development. Translational control is clearly a major level for coordinating the regulation of these genes during development, as is posttranslational stability of the ribosomal proteins and RNA splicing of the L1 gene. In addition to these levels of control under active investigation, a number of intriguing problems remain to be addressed in any detail. For example, the mechanisms that balance ribosomal protein production with subunit assembly in oocytes remain to be determined. Resolution of these events must also define the processes by which ribosomal proteins, upon synthesis in the cytoplasm, are first translocated to the nucleus and subsequently to the nucleolus for subunit assembly. Functional approaches in which these genes are assayed for accurate developmental control in microinjected oocytes and fertilized eggs will undoubtedly provide information on the synthesis of this eukaryotic organelle and the signals responsible for altering these processes at different developmental stages.