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Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, SC 29425; and
Department of Microbiology, University of Mississippi Medical Center, Jackson, MS 39216
The Ig heavy chain enhancer of the channel catfish
(Ictalurus punctatus) has an unusual position
and structure, being found in the 3' region of the µ gene and
containing eight functional octamer motifs of consensus (ATGCAAAT) and
variant sequences. The presence of multiple octamer motifs suggests
that an Oct2 homologue may play an important role in driving expression
of the Ig heavy chain locus in a teleost fish. To test this hypothesis,
two catfish Oct2 cDNAs (
and ß) were cloned by screening a catfish
B cell cDNA library. Catfish Oct2 and ß isoforms are derived by
alternative RNA splicing; as determined by Southern analysis, Oct2 is a
single copy gene. In comparisons with mammalian Oct2, the catfish Oct2
isoforms show high sequence conservation in their N-terminal regions
and POU domains, but extensive divergence in their C-terminal regions.
Catfish Oct2 and ß are tissue restricted, bind both consensus and
variant octamer motifs, and activate transcription in both catfish and
murine cells. In contrast, mouse Oct2 activated transcription in mouse
but not catfish cells. Catfish Oct2 ß is a more potent
transcriptional activator than Oct2 . In transient expression
assays, catfish Oct2 ß showed a marked preference for the octamer
variant, ATGtAAAT, which occurs twice in the catfish enhancer. Mouse
Oct2 also showed increased activity with the variant octamer when
tested in mouse B cells. Gel-shift analysis competition assays
indicated that catfish Oct2 binds the consensus octamer motif with an
apparently higher affinity than it does the variant motif.
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