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Many DNA-binding proteins that are involved in the differential regulation of gene expression are composed of multiple discrete modules. Association of the homeobox-encoded helix-turn-helix DNA-binding motif with conserved modules, such as the paired box or the POU domain, has led to the definition of structurally and functionally related subfamilies of regulatory proteins. The zinc finger, which is the second major nucleic acid-binding motif characterized to date, defines large multigene families in higher eukaryotes; we have isolated more than 100 Xenopus finger protein-encoding cDNAs and in this study we show that at least 10 of these clones share extensive sequence homologies in a region of more than 200 amino acids in the N-terminal nonfinger portion of the predicted proteins, which is connected to variable finger clusters. We refer to this element as a finger-associated boxes (FAX) domain. Cross-hybridization with human genomic DNA indicates that the finger-associated boxes domain is evolutionary conserved. Northern blot analysis shows that the corresponding genes are differentially expressed in the course of early Xenopus embryogenesis.
FIG. 1. Structural organization of FAX-domain proteins.
X. Iaevis oocyte- and gastrula-derived cDNA clones
predict finger-motif proteins that share extensive sequence
homologies in the N-terminal region (FAX domain). (A)
Structural organization in 4 of 10 examples ofFAX-domain
proteins. The limits of each element (designated A-H) are
defined by either the absence of specific segments from
individual protein sequences (i.e., element E present in
XlcOF-7.1 and XlcOF-8.4, but absent in XlcGF-48.2 and
XlcOF-29) or by their being separated by variable spacer
segments (i.e., spacer between elements C and D in
XIcOF-29). Finger repeats are represented by Qi and their
numbers are indicated. The predicted protein sequences of
XIcOF-7.1, XlcOF-29, and XlcGF-48.2 extend beyond the
first methionine in the A box, which is not preceded by an
in-frame termination codon. In consequence, possible
N-terminal extension sequences are indicated by dots. aa,
Amino acids. (B) Sequence comparison of FAX-domain
proteins. N-terminal sequences were aligned to XlcOF-
7.1; sequence identity is indicated by a dot and absence of
amino acids by a dash. Regions ofhomology are boxed and
labeled according to the scheme shown in A. Finger
elements in XlcOF-8.4 are organized as two hands (see A)
with additional H elements in the spacer segment separating
them; this region is designated 8.4i in the sequence
comparison. Multiple copies of the H element have been
aligned with the first H element in the XlcOF-7.1. Most of
the cDNA clones displayed encode open reading frames
with the first methionine located in the A element; XlcOF-
7.2 and -8.4 are likely to represent partial cDNA clones
lacking 5'-terminal sequence elements. For clarity and due
to limitations in space, only sequences of first finger
elements are shown. cDNA sequences have been deposited
in the GenBank data base.*
FIG. 2. Expression of FAX domain sequences in Xenopus embryogenesis
monitored by Northern blotting. Poly(A)+ RNA preparations
from Xenopus ovaries, oocytes, eggs, and early embryos
were probed with DNA fragments derived from either the 3'-terminal
portion (finger repeats) or the 5' region (FAX domain) of three
FAX-domain cDNA clones (shown in Fig. 1). The positions of
rRNAs are indicated. Identical blots were used with both probes for
XlcGF-48.2 and XlcGF-53.1. Lanes: 0, ovary; SO, small oocytes;
LO, large oocytes; E, eggs; B, blastula; G, gastrula; N, neurula; S,
somite stage; T, tadpole.
FIG. 3. Detection of sequences homologous to the Xenopus
FAX-domain-encoding region in the human genome. Southern blots
of EcoRI-digested genomic DNA from X. laevis (lane X), Homo
sapiens (lane H), and Escherichia coli (lane E) were hybridized with
the FAX-domain probe under low stringency. kb, Kilobase(s).
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