NEWS AND VIEWS

NATURE VOL i144 PHIL 1aR6

; aendon of molecular biologists. Their
| study may, inthe fong term, have relevance
| to the possibility of introducing the genes

responsible for N.-fixation (the nif genes)
mito the plastids, whether chlorophyllous
or not, of higher plants. They have also

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Fie. 2 Feolunonary progression of the metazoan bodyplan from odd-number radial svmmetry
“glide reflection” to true bilateral symmetry (M. Fedonkin). See teat ior detais

through
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gained attention as possibly useful models
for developmental molecular biologists
because many forms occur as simple un-
branched filaments with a maximum of
three cell types: heterocysts, the sites of N,
fixation: akinetes, which are perennial;
and vegerative cells, from which akinetes
and heterocysts develop*. Haselkorn and
co-workers now add to the scientific
altractions of cyanobacteria by showing

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3 o 2 praisal concerns the role of sott-bodied | gastral cavities, and so forth) actually in-
¢ ee a > organisms m the evolutionary radiation of | creases during growth. A possible insight | that they possess a so-far unique capacity
* 3 = s Su the early Cambrian. Many of the more | into metazoan evolution comes from fossils | to rearrange some of their N,-fixing genes, a eT
& S 3S 2 z 2 emgmatic fossils scem to represent body | such as the late Precambrian Dickinsoma | notably two that encode the major Set nD of Tr An
u e = 2 a a g gu plans that are not exhibited by any living | which at first sight are bilaterally sym- | components of nitrogenase. 4 , . OO .
i 2 s: - -5 2 a2 creature. The present-day Metazoa {multi- | metrical but on closer examination have The mechanism that allows the O.- re ane al tet % L were G ame
; 32° a Sz ae23 g cellular animals) are dominated by bilater- | different bumbers of segments on the left- | sensitive nitrogenase to function in ke . Th 4
‘ en 352292 3% ally symmetrical body plans, often with a | and right-hand sides. Fedonkin terms this | oxygenic cyanobacteria such as Anabaena bn Pht nag ce 2 Prey de
j gYe2 BS 23s = 32 2 degree of segmentation. The coeienterates. | phenomenon ‘‘glide reflection’’ and | was for long unknown. It was then C
: soe 3 a3 ye bare with concentric or radial symmetry. today | believes that such forms would not be | discovered?* that the peculiar, empty-
i 304 » maze 23 $54 constitute only a small part of the animal | dynamically stable and so evolved to | looking heterocysts’, which occur in most
: 33 SER 3GuEaS kingdom. But M. Fedonkin (USSR | become properly bilateral. He envisages a | Ny-fixing cyanobacteria, are the loci of oo
: acls SE BE552 Academy of Sciences, Moscow), studying | major evolutionary pattern, recorded in the | nitrogenase activity in air and in the light, weap arena.
i . the soft-bodied biota from the Vendian | Vendian rocks, of a gradual change from | and that by various biochemical modi- OF sg. Lon nns ,
{ a (about 650 Myr) of the White Sea Russian | the dominance of concentric and radial | fications, they provide an anaerobic micro- ey a NV Eaten om Tho “pod
{ 3 Platform and polar Siberia, concludes that | organization, through variable-order radial | environment in which nitrogenase is . i
j zg 3 = 70 per cent of animals were then radially | symmetry and glide reflection. to bilateral | synthesized and is functional. >: --r
— . 3 2 or concentrically symmetrical; those with | segmentation (Fig. 2). A. Seilacher (Univer- A drawback, until recently’, to the ng Han owe en
a a 2 3 3 = bilateral symmetry are often totally absent. | sitat Tiibingen) and others dispute this | detailed geneuc analysis of N.-fixation and oo q “$
= a < 3 ce. 2 3 Some of the more bizarre forms exhibit | view, finding no evidence that Dickinsonia heterocyst production in cyanobacteria has
a 2 a z S radial symmetries that today are very rare | had a mouth or a gut or any semblance of | been the fact that although mutants of
- ¥Y a z z & 3 i . or non-existent. Thus, an homogeneous | a metazoan digestive system. Consequently | cyanobacteria are readily obtainable, there 7
= a gE 2 aeave group with primary 3-fold symmetry is | they believe such extraordinary fossils rep- | has existed no good system for the transfer, '
a _ 2 az ees a recognized, and forms with 4- and 7-fold | resent not the dawn of metazoans but per- | in the jaboratory, of genes into hetero- i
D> OD eos 5 22 = % ordering are also common. A particularly | haps a completely different kingdom. {J | cystous cyanobacteria. Thus, genetic i
> a} 2 S32 § : important group, according to Fedonkin, analysis by complementation of cyano- :
a = Ny 2 533 a8 Bt is that in which the symmetry remains | Peter Gamobles is an assistant editor af Nature. | bacterial mutants has not been possible.
a es 3 3 g2ét > 7 * An alternative approach, used by :
: > BS Nitrogen fixation Haseikorn and colleagues, is to use the nif fy
7 vm a e 2 3 4248: Taae . . genes of the enteric bacterium ibid as t \
3 ' s & PERS BE D ff. t t b t probes for those of Anabaena. in |
nH & E 3 g » 23 5 ‘ 1 eren la Ing cyano ac CTla Klebsiefla there are 17 nif genes. ama : |
4 e <= S38 5h es ° into seven or eight transcriptional units an i
‘ Oo & § a aen2438 rearrange their nif genes arranged in a cluster occupying about 23 cy
{ ww A s&s sEe2 ge kilobases of DNA and located near the c |
\ 2 = = 2 3 2st z from William D.P. Stewart genes for histidine biosynthesis (see ref. 9). : i
an qa § 23 3 be The genes that encode the major |
~ B a BZ 5 “2 CYANOBACTERIA (blue-green algae) are | for example in parts of Antarctica where | nitrogenase components — the iron t !
aa c ES “2 2.2 oxygenic photosynthetic prokaryotes, | they may form ‘algal peat’, in hot desert | protein and the iron-molybdenum protein £ |
i > © SRE Dy 2 many strains of which fix N, and which | regions including parts of the Sahel, on | — are nif K, nif D and nif H. The iron i |
7 > 2 = 222 35 probably dominated the Earth’s biota | bare rock surfaces, in hot-spring regions | protein is composed of two identical ry
{ Z. yo 3 3 E 3 during the middle Precambrian, 2,500-570 | and in living stromatolites. Cyanobacteria | subunits, both encoded by nifH. The iron- . !
; £2 Jae 3 million years ago. On page 419 of thisissue, | are aiso important components of the | molybdenum protein contains one pair of :
j SOE SS J.W. Golden, S.J. Robinson and R. | marine phytoplankton and they, or more | identical subunits encoded by aif K and 7 i i/ Ji me
] Haselkorn report that, despite their ancient | probably their colourless analogues, are | another by nif D. Although the two pro- : Nadturs Y Sf
i origin, the N,-fixing photosynthetic | components of the microflora of the | teins do not fix N, alone, in combination ©
\ cyanobacteria possess a molecular | hydrothermal vent regions of the | they may do so in the presence of a source ‘ Su duce. Wf
\ complexity that belies their morphological | Galapagos Rift. In many parts of the | of reductant, Mg** and ATP, and in the 8 a ZA UNIVERSITY 1
{ simplicity’. world, but particularly in south-east Asia | absence of O, (see ref. 10). ° - — ~ _
4 Although long regarded as algae, mainly | where fertilizer nitrogen is not readily Haselkorn and his colleagues have p
' because of their pigment composition and | available, cyanobacteria are important | already shown that, while the nif K, Dand o
mode of oxygenic photosynthesis, | providers of biologically fixed nitrogen for | H genes are clustered in Klebsiella and in
cyanobacteria are probably more akin to | the growth of the rice plant?-. some other N,-fixing organisms. in DNA a
prokaryotic bacteria than to evkaryotic It is partly because of their unique ability | extracted from filaments of Anabaena 7
chlorophyilous plants, and are seldom still | to fix N, while photosynthesizing in the | 7120 the nif D and nif H genes are ‘a
classified as blue-green algae. They are | manner of higher plants that the | contigous but separated from aif K by ui
most noticeable in extreme environments, | cyanobacteria have recently aroused the | about 1} kilobases. The Chicago group has 'o