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

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.

Developmental expression and characterization of the gene encoding spore coat protein SP60 in Dictyostelium discoideum.

The complete coding sequence, upstream sequence and developmental expression of Dictyostelium discoideum AX2 spore coat protein gene SP60 is reported. The gene contains two exons, 154bp and 1121bp long, separated by a 119bp intron, and encodes a protein of 46,925 molecular weight plus a 23-amino-acid hydrophobic leader sequence. The N-terminus of the mature protein consists of four copies of a perfect hexapeptide repeat (GDWNNN). The central region is rich in cysteine residues, including four highly conserved cysteine-rich repeats with homology to 'EGF-like' repeats. The C-terminus is aspartate-rich and composed of multiple imperfect copies of a D(G/D)DYD repeat followed by several repeats of the tetrapeptide DNDW and derived sequences. A TATA box promoter motif juxtaposed to an oligo(dA) stretch lies 52bp upstream of the main transcriptional start site of the gene. Six AC-rich boxes occur in the region -327 to -556, all of which contain the consensus sequence CACAC. Two GC-rich boxes and a C-rich element (TTACCCCA) are also present upstream. Another open reading frame is positioned a short distance downstream of the SP60 gene in the opposite transcriptional orientation. Expression of the SP60 gene ceases upon disaggregation to single cells and cannot be restored by high levels of extracellular cAMP either alone or in combination with conditioned medium factors.