Biology Reference
In-Depth Information
4.1.8 Human genes whose origin preceded the divergence of plants and
animals
Some human genes have their counterparts in both plants and fungi implying that
they originated before the divergence of the three kingdoms (Doolittle
et al
., 1996).
Members of the
myb
gene family (
MYB
; 6q23.3-q24) are found in animals, plants,
fungi and even slime moulds (Lipsick, 1996; Rosinski and Atchley, 1998). Myb
proteins function as regulators of cell growth and differentiation by binding to
DNA and regulating gene expression. Since slime moulds such as
Dictyostelium
are
thought to have diverged before the main eukaryotic radiation which gave rise to
animals, plants and fungi ~1000 Myrs ago (Doolittle
et al
., 1996), the presence of
Myb proteins in this organism serves to date the emergence of this ancient regula-
tor of gene expression. Remarkably, despite this ancient origin, the Myb-related
proteins of
Drosophila
and
Dictyostelium
are still able to recognize and interact with
the same cognate DNA sequence as their vertebrate counterparts (Lipsick, 1996).
The High Mobility Group (HMG) proteins are another human gene family rep-
resented in both plants and fungi (Laudet
et al
., 1993). Two members of this fam-
ily,
HMG1
and
HMG2
, are present on human chromosomes 13q12 and 4q31
respectively. Members of the HMG protein superfamily are characterized by the
possession of one or more HMG boxes, each of which comprises ~80 amino acid
residues and is capable of interacting with DNA. Both the actins (Hennessey
et
al
., 1993) and the CCAAT-specific transcription factor, NF-Y (Li
et al
., 1992) have
their counterparts in plants and fungi whilst the annexins (Morgan
et al
., 1998)
and the cystatins (Rawlings and Barrett, 1990) are present in plants but not fungi.
4.1.9 Human genes whose origin preceded the divergence of prokaryotes
and eukaryotes
Would it be too bold to imagine that in the great lengths of time, since the
earth began to exist, perhaps millions of ages before the commencement of the
history of mankind, would it be too bold to imagine that all the warmblooded
animals have arisen from one living filament which the Great First Cause
endued with animality and thus possessing the faculty of continuing to
improve by its own inherent activity and of delivering down those improve-
ments by generation to its posterity, world without end.
Erasmus Darwin
Zoonomia
(1794)
Genes that are common to prokaryotes and eukaryotes must have arisen more
than 2000 Myrs ago before the divergence of the two groups (Doolittle
et al
., 1996).
A number of human genes fit into this category. Hexokinase is one of the best
characterized examples with sequences available from bacteria, yeast, plants and
vertebrates (three hexokinase genes
HK1
,
HK2,
and
HK3
exist in humans on
chromosomes 10q22, 2p12 and 5q35 respectively). A proposed multi-kingdom
phylogeny of the hexokinase gene is shown in
Figure 4.1
and depicts a series of
duplication and fusion events that must have occurred during its evolution
(Cardenas
et al
., 1998). The matrix metalloproteinases that play an important role
in tissue remodelling and wound healing have homologues in plants, animals and
bacteria (Massova
et al
., 1998).
