Diverse splicing mechanisms fuse the evolutionarily conserved bicistronic MOCS1A and MOCS1B open reading frames

Abstract

Molybdenum is an essential cofactor in many enzymes, but must first be complexed by molybdopterin, whose synthesis requires four enzymatic activities. The first two enzymes of this pathway are encoded by the MOCS1 locus in humans. We describe here a remarkably well-conserved novel mRNA splicing phenomenon that produces both an apparently bicistronic MOCS1AM-OCS1B transcript, as well as a distinct class of monocistronic transcript. The latter are created by a variety of splicing mechanisms (alternative splice donors, alternative splice acceptors, and exonskipping) to bypass the normal termination nonsense codon of MOCS1A resulting in fusion of the MOCS1A and MOCS1B open reading frames. Therefore, these "no-nonsense" transcripts encode a single bifunctional protein embodying both MOCS1A and MOCS1B activities. This coexpression profile was observed in vertebrates (human, mouse, cow, rabbit, opossum, and chicken) and invertebrates (fruit fly and nematode) spanning at least 700 million years of evolution. Our phylogenetic data also provide evidence that the bicistronic form of MOCS1 mRNA is likely to only produce MOCS1A protein and, combined with Northern analyses, suggests that MOCS1B is translated only as a fusion with MOCS1A. Taken together, the data presented here demonstrate a very highly conserved and physiologically relevant dynamic splicing scheme that profoundly influences the protein-coding potential of the MOCS1 locus.

New Ways of Initiating Translation in Eukaryotes?

“…The original cDNA revealed an ORF for MOCSIA upstream from the ORF for MOCS1B, with an intercistronic gap of 18 nt (61). Follow-up studies, however, suggest that this mRNA produces only the upstream MOCS1A protein and that MOCS1B is translated as a fusion protein from a spliced transcript (21). Another potential candidate is the dicistronic SNRPN mRNA (20).…”

Mutations in the molybdenum cofactor biosynthetic genesMOCS1, MOCS2, andGEPH

“…These isoforms have been described in detail by Gross-Hardt and and explain the existence of two different 5 0 sequences for MOCS1A that have been entered in the GenBank (AJ224328 and AF03474). Alternative splicing around exon 9 of the MOCS1 gene, described in detail by Gray and Nicholls [2000], also generates three isoforms. The originally described full-length cDNA from this gene contains two consecutive ORFs separated by a stop codon located within exon 9 [Reiss et al, 1998b].…”

Mechanistic Studies of Human Molybdopterin Synthase Reaction and Characterization of Mutants Identified in Group B Patients of Molybdenum Cofactor Deficiency

“…The MOCS1 locus encodes two proteins homologous to Escherichia coli MoaA and MoaC that are needed for the formation of precursor Z, which were shown to have an unusual bicistronic structure with open reading frames for both MOCS1A and MOCS1B in a single transcript (5,6). Similarly, the MOCS2 locus encodes the two subunits of MPT synthase and has been shown to be bicistronic with overlapping reading frames encoding MOCS2A and MOCS2B, the congeners of E. coli MoaD and MoaE (7).…”