Molecular organization of developmentally regulated Dictyostelium discoideum ubiquitin cDNAs

Dictyostelium discoideum ubiquitin mRNAs are regulated in a complex fashion during spore germination and multicellular development. Species of mRNA of 1900, 1400, 1100, 840, 580, and 500 nucleotides (nt) are found which are expressed differentially during different stages of development. DNA blot analysis indicates that ubiquitin genes constitute a multigene family of at least six genes. cDNAs representing all the ubiquitin mRNA transcripts were isolated and sequenced. The Dictyostelium mRNAs are organized as tandem repeats of the 76 amino acid ubiquitin unit (228 nt). We isolated one cDNA containing seven of these tandem repeats, and two different five- and three-repeat cDNAs. In addition, 2 cDNAs containing a single ubiquitin repeat fused at its 3' end to an unrelated 52 and 78 amino acid extension were identified. There is a remarkable similarity in the sequences of the non-ubiquitin extensions among yeast and mammalian counterparts. The extensions are very basic, containing approximately 30% lysine/arginine. Another common feature of these proteins is the presence of a common structural motif containing cysteine residues at conserved positions, suggesting a metal binding domain that matches a consensus sequence of Xenopus transcription factor TFIIIA and other nucleic acid binding proteins. The characterization of ubiquitin cDNAs and genomic sequences in D. discoideum now makes the understanding of its developmental regulation feasible.