The Nuclear RPL4 Gene Encodes a Chloroplast Protein That Co-purifies with the T7-like Transcription Complex as Well as Plastid Ribosomes

Abstract

We have cloned and sequenced the cDNA and the gene coding for plastid ribosomal protein L4 (RPL4) from two higher plant species, spinach and Arabidopsis thaliana. Ribosomal protein L4 is one of the ribosomal proteins for which extraribosomal functions in transcriptional regulation has been demonstrated in prokaryotes. Sequence comparison of the two plant cDNAs and genes shows that the RPL4 gene has acquired a remarkable 3 extension during evolutionary transfer to the nuclear genome. This extension harbors an intron and codes for a glutamic and aspartic acid-rich amino acid sequence that resembles highly acidic C-terminal tails of some transcription factors. Co-purification of ribosomal protein L4 with plastid RNA polymerase and transcription factor CDF2 using different purification protocols as well as the surprising amino acid sequence of the L4 protein make it a likely candidate to play a role in plastid transcriptional regulation.Higher plant plastid ribosomes are closely related to those found in eubacteria, reflecting the endosymbiotic origin of chloroplasts. They contain about 54 -75 ribosomal proteins, depending on the plant species (1-3). The complete sequencing of several higher plant plastid genomes has shown that about one third of the plastid ribosomal (r) 1 proteins are encoded by the plastid genome itself (4 -7). The remaining two thirds are probably encoded by the nuclear genome, and it is thought that the genes have been transferred from the plastid genome to the nucleus during evolution. From these nuclear-encoded plastid r-proteins, only few have been characterized by their cDNAs and/or by their genes (see Harris et al. (1) and references therein).During the last 10 years ribosomal proteins became of special interest because it was shown in a number of cases that they have extraribosomal functions apart from the ribosome and protein biosynthesis (for review, see Wool (8)). These extraribosomal functions concern basic cellular processes like replication, transcription, RNA processing, translation, and DNA repair. Specific functions in transcription have been reported for r-proteins S10 (9 -12), L4 (13-15) and S14 (16). In all three cases, the regulation by r-proteins concerns the expression of ribosomal components (rRNA, operon S10, and mRNA for rprotein S14).The plastid genome is transcribed by two different RNA polymerases. One is nuclear-encoded, T7-like, and especially active during early phases of plastid development. This polymerase is more or less specified for transcription of plastid housekeeping genes, encluding subunits of the second RNA polymerase, which is plastid-encoded, prokaryotic-like, and transcribes preferentially photosynthesis-related genes during later phases of plastid development (17-21). The transcriptional activity of the prokaryotic-type RNA polymerase is regulated by nuclear-encoded, prokaryotic-type, and sigma-like factors (22, 23).Our group has been working for several years on the expression of rrn transcription in spinach plastids. Transcription of the rrn opero...

Abscisic acid and sucrose increase the protein content in date palm somatic embryos, causing changes in 2-DE profile

“…In the Populus dormant terminal buds, a total of six phosphorylation sites were identified on eight ribosomal P-proteins during the maintenance of dormancy (Liu et al, 2010). Trifa et al (1998) have showed that the L4 protein in Arabidopsis and spinach was implicated in the regulation of plastid rDNA expression and transcription. Many ribosomal proteins have a second function apart from the ribosome (Wool, 1996).…”

The transcription machineries of plant mitochondria and chloroplasts: Composition, function, and regulation

“…While CDF2-A might repress transcription initiation by PEP at the rrn16 P1 promoter (termed P2 in spinach), CDF2-B possibly binds NEP and initiates specific transcription from the rrn16 Pc promoter. In addition, a role of RPL4 (plastid ribosomal protein L4, encoded by the nuclear Rpl4 gene) has been discussed to be involved in NEP transcription, since it co-purifies with the T7-like transcription complex in spinach (Trifa et al, 1998). The bacterial L4 has extra-ribosomal functions in transcriptional regulation (Zengel et al, 1980).…”

Eukaryotic transcription factors in plastids — Bioinformatic assessment and implications for the evolution of gene expression machineries in plants

“…The identity of these proteins, however, remained elusive so far. In addition to these factors, a number of nucleoid-binding proteins such as MFP1, DND41, PEND, DPC68, SiR, RPL4 and ET1 (Sato et al, 1993;Nakano et al, 1997;Trifa et al, 1998;Sato et al, 2001;Chi-Ham et al, 2002;Jeong et al, 2003; da Costa e Silva et al, 2004) were identified. Most of them seem to be involved in the structural integrity and membrane association of nucleoids.…”