Deoxyribonucleic acid base composition of geographically diverse strains of Leucothrix mucor

Leucothrix mucor is a colorless organism related to the blue-green algae (R. Harold and R. Y. Stanier, Bacteriol. Rev. 19:49, 1955; E. G. Pringsheim, Bacteriol. Rev. 21:69, 1957), which is widespread in the marine environment, occurring primarily as an epiphyte on algae (T. D. Brock, Limnol. Oceanog., in press). This organism is recognized and taxonomically defined by a set of morphological properties which include: (i) nonmotile colorless filaments (2 to 4 ,u in diameter) of varying lengths, each filament composed of many cells; (ii) under certain conditions cells of the filaments round up and separate into individual cells called gonidia which exhibit gliding motility; (iii) production by gonidia of holdfast material, permitting attachment to surfaces or to other gonidia; and (iv) mutual attraction of gonidia at high gonidial density, leading to the formation of characteristic rosettes. Gonidial formation occurs predominantly under conditions where growth is slow, such as in media of low organic nutrient concentration (E. G. Pringsheim, Bacteriol. Rev. 21:69, 1957) or at low temperatures (T. D. Brock, unpublished data). Another morphological feature found in all isolates is knot production (T. D. Brock, Science 144:870, 1964), the knots forming test under conditions promoting rapid and prolific growth. Because of the distinctive morphology of L. mucor and because of its occurrence as an algal epiphyte, it is relatively easy to recognize the organism directly in natural collections. A fairly extensive survey of marine habitats (T. D. Brock, Limnol. Oceanog., in press) has revealed that L. mucor is found predominantly in temperate oceanic waters in regions where there is good aeration due to tidal current or wave action. Isolates have thus been obtained from geographically diverse areas, and these isolates have proved remarkably similar in morphological and physiological properties. All isolates, for instance, show similar nutrition, salt requirements, pH tolerance, and temperature tolerance, and the main difference seen so far is in the relative degree of conversion of filaments to gonidia. It was therefore of interest to study the deoxyribonucleic acid (DNA) base composition of these strains, since this characteristic has proved useful in uncovering differences between otherwise closely related bacteria (J. Marmur et al., Ann. Rev. Microbiol. 17:329, 1963). In addition to the strains isolated by the senior author, isolates were obtained from other people who had worked with L. mucor. Unfortunately, the strains of E. G. Pringsheim are no longer available (personal communication to T. D. Brock). All strains were grown in shaken flasks in the following medium: NaCl, 11.7 g; MgC92-6H20, 5.35 g; Na2SO4, 2 g; CaCl2.2H20, 0.75 g; KCl, 0.35 g; tris(hydroxymethyl)aminomethane, 0.5 g; Na2HPO4, 0.05 g; monosodium glutamate, 10 g; water, 1,000 ml; pH 7.6. Each strain was grown in 1 liter of medium dispensed in four 500-ml flasks for 2 days at 25 C; the cells were centrifuged and washed twice in the above medium without monosodium glutamate, and the final pellet was frozen at -70 C until needed. Between 3 and 6 g (wet weight) of cells were obtained per liter of culture medium, depending on the strain. The pellet was suspended in 50 ml of saline-ethylenediaminetetraacetic acid, and 10 to 15 mg of crystalline lysozyme was added. After 1 hr at 37 C, lysis was complete and the suspension was highly viscous. The DNA isolation and purification then followed the procedure of J. Marmur (J. Mol. Biol. 3:208, 1961), with elimination of the final isopropanol precipitation. The buoyant densities of the DNA preparations were determined by CsCl density gradient centrifugation according to the method of C. L. Schildkraut, J. Marmur, and P. Doty (J. Mol. Biol. 4:430, 1962), with Bacillus subtilis bacteriophage SP8 DNA as a density reference (1.742 g/ml). The buoyant densities and the calculated guanine plus cytosine (GC) contents of the DNA