Cellular Nucleic Acid Binding Protein Binds a Conserved Region of the 5 0 UTR of Xenopus laevis Ribosomal Protein mRNAs Livio Pellizzoni 1 , Francesco Lotti 1 , Bruno Maras 2 and Paola Pierandrei-Amaldi 1 * 1 Istituto di Biologia Cellulare CNR, Viale Marx 43 00137, Roma, Italy 2 Dipartimento di Scienze Biochimiche and Centro di Biologia Molecolare CNR Universita Á ``La Sapienza`` Piazzale Aldo Moro 5 00185, Roma, Italy Vertebrate ribosomal protein mRNAs share structural features in the 5 0 untranslated region implicated in the control of their translation. A pyri- midine tract, at the 5 0 end, is considered the common cis-acting element, but the control requires also the integrity of the conserved downstream region. These sequences interact in vitro with proteins, which may rep- resent the trans-acting factors for a common regulation. The protein that binds the pyrimidine tract has been identi®ed as La and its binding in vitro depends on interaction with a protein factor. In the present study, by puri®cation, microsequencing and immunoprecipitation analy- sis we have identi®ed the protein that interacts with the region down- stream of the pyrimidine tract as the Xenopus laevis cellular nucleic acid binding protein (CNBP). The interaction of this protein with the con- served region of various ribosomal protein (rp)-mRNAs suggests a class- speci®c recognition. The binding of CNBP to the target region requires the assistance of a protease-sensitive factor, that dissociates after complex formation. Some evidence suggests that this may be the same factor that assists the binding of La to the 5 0 untranslated region (UTR) of the rp- mRNAs. Considering that CNBP and La come in contact with two typi- cal regions of the 5 0 UTR, essential for regulation, their interaction with the assisting factor may exert a modulating activity on the translational control of ribosomal protein mRNAs. # 1997 Academic Press Limited Keywords: cellular nucleic acid binding protein; ribosomal protein mRNA; RNA-protein interaction; translational control; Xenopus laevis *Corresponding author Introduction In vertebrates the synthesis of ribosomal proteins is controlled by regulation at various levels (Pier- andrei-Amaldi & Amaldi, 1994). One of these is the translational level that works by changing the ef®- ciency of utilization of the rp-mRNAs (mRNAs speci®c for r-proteins) in response to the cellular requirement for ribosomes. This translational regu- lation is clearly observed during development and in cultured cells under different growth conditions (Geyer et al., 1982; Pierandrei-Amaldi et al., 1982, 1985; Agrawal & Bowman, 1987; Loreni & Amaldi, 1992). It has been shown in Xenopus and in other systems that the 5 0 untranslated region (5 0 UTR) contains the cis-acting elements of the translational regulation (Mariottini & Amaldi, 1989; Levy et al., 1991; Hammond et al., 1991; Pierandrei-Amaldi & Amaldi, 1994). Accordingly, all mRNAs for differ- ent r-proteins share similarities in their 5 0 UTR. In particular, a pyrimidine tract, located at the 5 0 end of these mRNAs, is necessary to exert the transla- tional control in Xenopus and in mammals (Levy et al., 1991; Kaspar et al., 1992), but is not suf®cient, since the region downstream in the 5 0 UTR is also important for the control (Avni et al., 1994). More- over, it has been shown in reciprocal transfection experiments that the same cell signals control the selective translation of Xenopus and mammal rp-mRNAs, suggesting the conservation of factors able to recognize the rp-mRNA 5 0 UTRs of both species (Avni et al., 1994). This type of 5 0 UTR is present not only in rp-mRNAs, but also in mRNA for other proteins related to the synthesis and func- Abbreviations used: PMSF, phenylmethylsuolfonyl¯uoride; r-protein, ribosomal protein; rp-mRNA, ribosomal protein mRNA; UTR, untranslated region; CNBP, cellular nucleic acid binding protein. J. Mol. Biol. (1997) 267, 264±275 0022±2836/97/120264±12 $25.00/0/mb960888 # 1997 Academic Press Limited