Integron-encoded integrases recognize two distinct types of recombination site : attI sites, found in integrons, and members of the 59-base element (59-be) family, found in the integron-associated gene cassettes. The class 1 integron integrase, IntI1, catalyses recombination between attI1 and a 59-be, two 59-be, or two attI1 sites, but events involving two attI1 sites are less efficient than the reactions in which a 59-be participates. The full attI1 site is required for high-efficiency recombination with a 59-be site. It is 65 bp in length and includes a simple site, consisting of a pair of inversely oriented IntI1-binding domains, together with two further directly oriented IntI1-binding sites designated strong and weak. However, a smaller region that contains only the simple site is sufficient to support a lower level of recombination with a complete attI1 partner and the features that determine the orientation of attI1 reside within this region. An unusual reaction between the attI1 site and a 59-be appears to be responsible for the loss of the central region of a 59-be to create a potential fusion of two adjacent gene cassettes.

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A chlorophyll c-like pigment, similar to magnesium-3,8-divinyl pheoporphyrin as monomethyl ester, has been isolated from Prochloron sp. obtained from five species of didemnid ascidians from the Great Barrier Reef, Australia, and from Palau, Micronesia. The pigment represents 4-15% of the total chlorophyll content and is shown to function in a light-harvesting pigment protein complex of Prochloron. The observation that all ofthe major chlorophylls (a+b+c) function in a light-harvesting role in Prochloron and possibly in other prochlorophytes is discussed in terms of the phylogeny of the prochlorophytes.The discovery of Prochloron sp., a symbiotic prokaryotic alga with chlorophyll (Chl) b and Chl a (1, 2), stimulated debate as to the origin of chloroplasts. Discussion has continued to the present time, fueled by the recent discoveries of two other prochlorophytes-the filamentous freshwater alga Prochlorothrix hollandica (3) and the small, unicellular, free-living planktonic alga Prochlorococcus marinus (4). Some view the prochlorophytes as allied to the ancestors of green chloroplasts (4,5), while others consider them as members of the cyanobacteria, which have lost phycobiliproteins and gained Chl b, probably independently, in each group (6-10). The presence of a Chl c-like pigment, tentatively identified as magnesium 3,8-divinylphaeoporphyrin a5 monomethyl ester (MgDVP), has been claimed for Prochlorococcus (4,11), although only the peak wavelengths of the natural and demetallated pigment were provided (11). It also has been claimed that MgDVP occurs in Prochlorothrix hollandica (11) in low concentrations. Here we present evidence, based on the spectra of pigments resolved by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC), that a Chl c-like pigment tentatively identified as MgDVP occurs in Prochloron in concentrations that are 4-15% of the total Chl. Moreover, the pigment is shown to be located in a light-harvesting Chl pigment-protein complex (LHC) and to act as a lightharvesting pigment. MATERIALS AND METHODSProchloron spp. were isolated from five didemnid ascidians (Table 1) at One Tree Reef, Capricorn Section, Great Barrier Reef, as described (15).Analysis ofthe pigments ofProchloron was done on site by reverse-phase HPLC using a Waters 600E quaternary pump system with a Waters 990 photodiode array detector and a Shimadzu RF-551 spectrofluorimeter. The reverse-phase system was used with a C18 column (3.9 x 150 mm Waters Novapak 36975; 4-,um particle size, spherical end-capped material with a low carbon load), and gradient elution with acetonitrile/methanol/water (method 1) was applied as folThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.lows: initial conditions, 95% solvent A/5% solvent B; linear gradient to 1% A/99% B in 15 min; and then held at 1%A/99%o B for 15 min [solvent A = 50% water/25% acetonitrile...

Heat shock proteins are ubiquitous and highly conserved. Recently they have become implicated in the import of proteins into organelles. All the heat shock genes characterized to date, however, are known or assumed to be encoded in the nuclear genome even if the corresponding protein can be localised in the mitochondrion or chloroplast. In contrast, we identify here an hsp70 gene in the unicellular chromophytic alga Pavlova lutherii which is located on the chloroplast genome. Localisation of this gene to the chloroplast chromosome is confirmed by Southern blot analysis and pulse-field gel electrophoresis which also reveals that the length of the P. lutherii chloroplast chromosome is 115 kb. We compare the predicted protein of this hsp70 gene with that of maize and of the analogous proteins in the prokaryotic organisms Escherichia coli and Synechocystis PCC6803. The greatest identity is found with the cyanobacterium Synechocystis PCC6803.