A developmentally regulated gene encodes the dictyostelium homolog of yeast ribosomal protein S4 and mammalian LLRep3 proteins

Abstract

We report the sequence and expression of a single-copy gene from Dictyostelium discoideum which encodes the homolog of yeast ribosomal protein S4, a protein located on the small ribosomal subunit and known to play an important role in maintaining translational fidelity. Over a highly conserved central region, the Dictyostelium protein has 78% sequence similarity to the yeast protein and 83% sequence similarity to mammalian S4 protein homologs, the LLRep3 proteins. The Dictyostelium gene encodes a polypeptide 28,717 Da in size and hence this ribosomal protein has been named rp29. The N-terminal sequence of the Dictyostelium rp29 protein is extended by 61 amino acids and 14 amino acids compared to the mammalian and yeast proteins, respectively, and the C-terminus is correspondingly 15 amino acids or 2 amino acids shorter. Although the coding region of the rp29 gene is present on a single exon, a 157bp intron interrupts the 5' untranslated region and unusually contains four direct repeats of the sequence TCAATCT. The gene is expressed maximally during vegetative growth but a second peak of expression also occurs late in development which is restricted to prestalk cells; rp29 is the first Dictyostelium ribosomal protein gene reported which shows prestalk-specific developmental expression. During each round of expression, only a single 0.9kb transcript is produced which is similar in size to the yeast S4 ribosomal protein transcript (0.8kb) but markedly smaller than the mammalian LLRep3 mRNA (1.7kb) due to a much shorter 5' untranslated region.

Ribosomal protein gene expression is cell type specific during development in Dictyostelium discoideum

“…The gene contains two introns; the first starts at position +28, immediately adjacent to the codon for the first amino acid, the initiator methionine, and ends at +314. This placement of an intron which isolates the initiator methionine codon and 5' non-translated leader sequence in a separate exon has been observed in Dictyostelium rprotein genes rp-S17 and rp-12 but not in rp-S9, rp-S4 or rp-L3 [7,24,50,62,66]. Like the first introns in these other r-protein genes, the first intron in the rp-L11 gene is both unusually long (286 bases) and unusually GC rich (75%) for Dictyostelium, where introns are usually only about 100-150 bases and 80-90% A+T [28].…”

“…Like the first introns in these other r-protein genes, the first intron in the rp-L11 gene is both unusually long (286 bases) and unusually GC rich (75%) for Dictyostelium, where introns are usually only about 100-150 bases and 80-90% A+T [28]. This intron, however, differs from the other four r-protein gene first intron sequences in that it does not contain a CAAT sequence within the intron [7,24,50,62,66]. The second intron from +538 to +604 is more typical of a Dictyostelium intron being only 66 bases in length and 87% A+T.…”

“…It is unlikely, however, that this poly-A addition site, is actually used as the C5 cDNA clone, contains an additional 30 bases of 3' sequence to position +1015, suggesting that there is a 3' non-translated region on the mRNA as well. Interestingly three of the other five sequenced r-protein genes (rp-12, rp-S4 and rp-S17) also have a poly-A addition site that overlaps the termination codon [7,24,50]. Comparison of the full predicted peptide sequence to the rat rp-L11 gene indicates that there is 73.6% identity and 93.6% similarity in a 174-amino-acid overlap.…”

“…The loss of these mRNAs from polysomes occurs within 20 min of removal of nutrients and is also dependent upon sequences in the 5' non-translated leader of the r-protein mRNAs [1,63,66]. The nature of the motif in the 5'NTL responsible for the exclusion of Dictyostelium r-protein mRNAs from polysomes may differ from the 5' TOP sequence found in higher eukaryotes since not all of the Dictyostelium rprotein mRNAs contain a 5' oligopyrimidine tract in their 5'NTL [9,24,50,62,64].…”

PT‐12, a putative ras‐activated proliferation‐dependent gene, is expressed in patellar tendon and not in anterior cruciate ligament

“…Human S2 is overexpressed by more than 25 times in Ras-transformed highly proliferating cells (8) and is upregulated during chemically induced oral carcinogenesis in the hamster model (29). Furthermore, a homologue of PT-12, rp29, is overexpressed in highly proliferating cells during development in Dictyyostelizmz discoideurn (27). This unique observation could make possible the use of specific gene markers to study changes in genc expression in patellar tendon cells after reconstructive surgery of the anterior cruciate ligament.…”