A) Polycistronic eukaryotic transcripts are rare. A recent article makes a
strong case for at least one such transcript in mammals and provides
references for others:
Gray, T. A., S. Saitoh, et al. (1999). "An imprinted, mammalian bicistronic
transcript encodes two independent proteins." Proc. Natl. Acad. Sci. U.S.A.
96(10): 5616-5621.
Polycistronic transcripts are common in prokaryotes but rare in eukaryotes.
Phylogenetic analysis of the SNRPN (SmN) mRNA in five eutherian mammals
reveals a second highly conserved coding sequence, termed SNURF (SNRPN
upstream reading frame). The vast majority of nucleotide substitutions in
SNURF occur in the wobble codon position, providing strong evolutionary
evidence for selection for protein-coding function. Because SNURF-SNRPN maps
to human chromosome 15q11-q13 and is paternally expressed, each cistron is a
candidate for a role in the imprinted Prader-Willi syndrome (PWS) and PWS
mouse models. SNURF encodes a highly basic 71-aa protein that is
nuclear-localized (as is SmN). Because SNURF is the only protein-coding
sequence within the imprinting regulatory region in 15q11-q13, it may have
provided the original selection for imprinting in this domain. Whereas some
human tissues express a minor SNURF-only transcript, mouse tissues express
only the bicistronic Snurf-Snrpn transcript. We show that both SNURF and
SNRPN are translated in normal, but not PWS, human, and mouse tissues and
cell lines. These findings identify SNURF as a protein that is produced
along with SmN from a bicistronic transcript; polycistronic mRNAs therefore
are encoded in mammalian genomes where they may form functional operons.
B) At least in Saccharomyces, initiation at downstream start codons in
transcripts containing more than one long ORF is normally not efficient.
However, there appear to be circumstances that support internal initiation:
Paz, I., L. Abramovitz, et al. (1999). "Starved Saccharomyces cerevisiae
cells have the capacity to support internal initiation of translation." J.
Biol. Chem. 274(31): 21741-21745.
Internal initiation of translation, whereby ribosomes are directed to
internal AUG codon independently of the 5' end of the mRNA, has been
observed rarely in higher eucaryotes and has not been demonstrated in living
yeast. We report here that starved yeast cells are capable of initiating
translation of a dicistronic message internally. The studied element that
functions as an internal ribosome entry site (IRES) is hardly functional or
not functional at all in logarithmically growing cells. Moreover, during the
logarithmic growth phase, this element seems to inhibit translation
reinitiation when placed as an intercistronic spacer or to inhibit
translation when placed in the 5'- untranslated region of a monocistronic
message. Inhibition of translation is likely due to the putative strong
secondary structure of the IRES that interferes with the cap-dependent
scanning process. When cells exit the logarithmic growth phase, or when
artificially starved for carbon source, translation of the IRES-containing
messages is substantially induced. Our findings imply that the capacity to
translate internally is a characteristic of starved rather than vegetatively
growing yeast cells.
Hope this helps.
Matt
____________________________________________________
Matthew S. Sachs
Department of Biochemistry and Molecular Biology
Oregon Graduate Institute of Science and Technology
20,000 NW Walker Road
Beaverton, OR 97006-8921
503 748-1487 phone
503 748-1464 fax
msachs at bmb.ogi.edu
-----Original Message-----
From: owner-yeast at hgmp.mrc.ac.uk [mailto:owner-yeast at hgmp.mrc.ac.uk]On
Behalf Of Carl Blobel
Sent: Tuesday, December 28, 1999 10:05 AM
To: nobody at net.bio.net
Subject: Polycistronic pombe mRNA
Hello everyone.
In screening a S. pombe cDNA library, I have isolated several clones
which contain an ORF I am interested in. However, they are all
downstream of a second ORF. Based on a comparison with known genomic
sequences, the cDNAs have been properly spliced and do not represent the
ligation of two seperate cDNAs. I have been unable to find any
published reports of polycistronic mRNAs and was wondering if anyone has
had any similar observations. Thanks!
Johannes Schlondorff
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