_31_0______NEWSANDVIEWS------N_A_ru_RE_v_oL_.304_28_JU_Lv_1983 are required for splicing at a downstream recognition that could apply, if only in found in a variety of pools and lakes, A-G. It will be interesting to see whether altered form, to higher organisms. D ranging from oligotrophic to eutrophic, this analogy will extend further. One diffi­ and have found only two species which culty is that A-G has been seen at positions S.M. Mount is in the Department ofMolecular showed overnight accumulation of acids. -5, -4 in two vertebrate intronsll,14 . Biophysics and Biochemistry, Yale University These are aquatica, a of the School of Medicine, 333 Cedar Street, New A second possibility, advocated by Haven, Connecticut 06510. margins of temporary ponds, and the Rosbash's laboratory, is that UACUAAC, shoreweed, Littorella uniflora, found present in the mRNA precursor, plays a I. Langford, C. &Gallwitz, D. Ce//33, 519 (1983). mainly along the gravelly edges of nutrient­ part in 5' splice-site recognition. They have 2. Ng, R. & Abelson, J. Proc. natn. Acad. Sci. U.S.A. 77, poor lakes with fluctuating water levels. 3912 (1980). The authors interpreted their results as sup­ observed that UACUAAC is itself a site of 3. Gallwitz, D. &Sures, I. Proc. natn. Acad. Sci. U.S.A. 11, cleavage. In trying to understand this result 2546 (1980). port for the hypothesis that aquatic plants 4. Teem, J.L.& Rosbash, M. Proc. natn. Acad. Sci. U.S.A. can increase their efficiency of carbon they noticed that the 5' end of the Ul RNA, 80 (in the press). which is proposed to recognize 5' splice 5. Leer, R.J.eta/. Nucleic Acids Res, 10, 5869(1982). utilization in this way under conditions sites15·17 , is homologous to UACUAAC 6. Kaufer, N.F. eta/. Nucleic Acids Res. 11, 3123 (1983). where carbon availability is likely to be 7. Pikielny, C. Personal communication. (the Ul sequence is UACUUAC). Thus, 8. Mount, S.M. Nucleic Acids Res. 10,459 (1982). growth limiting. they reasoned that UACUAAC might 9. Breathnach, R. & Chambon, P. A. Rev. Biochem. 50, From a taxonomic and an evolutionary 349 (1981). point of view, one of the most interesting interact with the 5' splice site, the cleavage 10. Breathnach, R. et al. Proc. natn. Acad. Sci. U.S.A. 75, ofUACUAAC resulting in some way from 4853 (1978). groups in which such diurnal acid meta­ this interaction. This proposal is supported II. Beggs, J.D. eta/. NatureW, 835 (1980). bolism has emerged is the fern /soetes. The 12. Langford, C. el al. Proc. natn, Acad. Sci. U.S.A. 80, 1496 by their observation that UACUAAC (1983). mechanism seems to be very uncommon 13. van het Schip, A.D. et al. Nucleic Acids Res. 11, 2529 among pteridophytes, having previously cleavage is inhibited in a 5' splice-site mu­ (1983). tant. 14. Maurer, R.A. et al. J. biol. Chem. 256, 10524 (1981). been reported only in two tropical ferns 9, Whichever possibility is upheld by later 15. Lerner, M.R. eta/, Nature 213,220 (1980). and it was first reported in Isoetes by 16. Rogers, J. &Wall, R. Proc. natn. Acad. Sci. U.S.A. 11, results, research on splicing in yeast should 1877 (1980). Keeley 1°. He found an overnight accumu­ elaborate a mechanism of splice-site 17. Mount, S.M. et al. Cell 33, 509 (1983). lation of malic acid in the leaves of Isoetes howellii, which grows in seasonal pools Plant ecology produced by the winter and spring rains of California, which dry out during the sum­ mer drought. Keeley has since surveyed 11 other Isoetes species 11 (the genus contains Photosynthetic pathways in about 70 species in all) derived from loca­ tions as far apart as Guatemala, British aquatic plants Columbia and North Wales. All the species from Peter D. Moore were found to exhibit diurnal acid meta­ bolism, and Keeley has proposed 8 that it THE C 4 and crassulacean acid metabolism value. But although a C 4 mechanism is pre­ may be present in all members of the genus. (CAM) photosynthetic systems are sent, there is no evidence of nocturnal net Many of the species, like /. howellii, generally associated with terrestrial plants carbon uptake occurring in Elodea. become emersed during dry periods and the growing in habitats where water stress and Work on other aquatic plants, however, diurnal variations in leaf acid content then high temperature are frequently has shown that at least some species can become less marked. encountered 1·3. The efficiency of carbon enhance their carbon metabolism by night­ These studies are of interest and impor­ accumulation resulting from the elimina­ time fixation. For example, the North tance both in their further demonstration tion of photorespiration permits these American submerged water plant Scirpus of the taxonomically widespread nature of plants the luxury of closer stomata! control subterminalis has been shown to ac­ C4 and CAM mechanisms of carbon fixa­ leading to lower water losses. In the case of cumulate malate during the dark (at least in tion and also in that they indicate another CAM plants, stomata open for gaseous ex­ part derived from respiratory CO:z) which ecological advantage of such a mechanism change only at night when potential water accounted for 12 per cent of the net photo­ in an unexpected habitat. In this case, a losses are minimal. synthesis 6. daytime shortage of dissolved inorganic Some C 4 species are known from marine Whilst working with another aquatic carbon for use in photosynthesis in aquatic saline habitats such as estuaries and salt macrophyte, Hydrilla verticillata, Holo­ ecosystems has led to the evolutionary marshes, for example Spartina anglica4, day and Bowes 7 found a considerable selection of a biochemical pathway which but it is surprising to find that some of the variation in CO2 compensation point (the has previously been associated largely with biochemical features associated with this CO2 concentration below which a plant arid environments where its value relates to photosynthetic strategy have been des­ takes up less CO2 by photosynthesis than it economy in water loss rather than a general cribed recently in submerged freshwater gives out by respiration), which depended scarcity of carbon. D aquatic plants. One such plant is the Cana­ on growth conditions. Winter collections dian pondweed, Elodea canadensis5• had high compensation points and summer 14 Peter D. Moore is a Senior Lecturer in the When Elodea was supplied with CO2, 45 collections low ones. The plants with low Department of Plant Sciences, University of per cent of the label ended up in C 4 acids compensation points, when provided with London King's College, 68 Half Moon Lane, rather than C 3 compounds. 14CO2' showed a 60 per cent incorporation London SE24 9JF. One possible explanation of the occur­ of labelled carbon into the C 4 acids malate rence of such a pathway in an and aspartate, and a high activity of phos­ I. Woolhouse, H.W. End,avour2, 35 (1978). is the diurnal fluctuation in dissolved car­ phoenol pyruvate carboxylase, the first en­ 2. Stowe, l..G. &Teeri, J.A. Am. Nat. 112. 609 (1978). 3. Mooney. H.A .. Ehleringer, J. & Berry, J.A. Science bon dioxide in a freshwater body, especial­ zyme in the C 4 fixation pathway. Clearly 194. 322 (1976). ly if it contains a high density of aquatic these plants have the ability to vary the CO2 4. Long, S.P., lncoll, L.D. & Woolhouse, H.W. Nature 257,622 (1975). plants. Carbon dioxide concentration compensation point in response to environ­ 5. DeGroois. D. & Kennedy, R.A. Pl. Physiol. 59, 1133 would tend to be high during the night and mental demands by the facultative assump­ (1977). low during the day, when it would be in de­ tion of a C fixation system. 6. Beer, S. &Wetzel, R.G. Pl. Sci. lei/. 21, 199 (1981). 4 7. Holaday, A. &Bowes, G. Pl. Physiol. 65,331 (1980). mand for photosynthetic fixation. A Just how widespread C 4 fixation is 8. Keeley, J.E. & Morton, B.A. Photosynthetica 16. 546 mechanism whereby night fixation of CO2 among water plants is not yet known, but (1982). 8 9. Wong, S.C. &Hew C.S. Am. Fern J. 66. 121 (1976). could take place (as in CAM plants) could Keeley and Morton have recently con­ 10. Keeley, J.E. Am. J. Bot. 68. 420 (1981). therefore be of considerable selective ducted a survey of 30 species of aquatic II. Keeley, J.E. Am. J. Bot. 69. 254(1982).

0028-0836/ 83/ 300310,0ISOl .00 «:> 1983 Macmillan Journals Lid