Thirteen strains of a strict anaerobic, extreme thermophilic bacterium were isolated from soil samples of moderate temperature, from a sewage plant in Georgia, and from hot springs in Utah and Wyoming. They were identified as strains of Clostridium thermohydrosulfuricum. T'he guanosine + cytosine content (moles percent) was 37.6 (determined by buoyant density) and 34.1 (determined by melting temperature). All strains required a factor present in yeast extract or tryptone for growth. Growth characteristics were as follows: a pH range of 5 to 9, with the optimum between 6.9 to 7.5, in a temperature range of 40 to 780C, with the optimum at 68°C. The doubling time, when grown on glucose at temperature and pH optima, was 1.2 h. The main products of glucose fermentation were ethanol, lactate, acetate, CO2, and H2?. The fermentation was inhibited by H,. Formation of spores occurred easily on glucose-agar medium or when cultures growing at temperatures above 65°C were allowed to cool to temperatures below 55°C. C. thermohydrosulfuricum occurs widely distributed in the natural environmient. The diversity of thermophilic bacteria is great, and many aerobes that grow over 70°C have been described (18). However, among anaerobic bacteria, only Desulfoiibrio thermophilus (17), Methanobacterium thermoautotrophicum (26), Clostridium thermohydrosulfuricum (5, 7), and two not well-characterized clostridia species (10) have been reported to grow and multiply at 70°C or higher. C. thermohydrosulfuricum was first described by Klaushofer and Parkkinen (7), who isolated it from extraction juices obtained in an Austrian sugar beet factory. The neotype, strain E100-69 (13), is a reisolate also obtained from Austrian sugar beet juices (5), which until now was the only reported source of this extreme thermophilic Clostr-idium. Several studies of the surface structure of this bacterium have been conducted (5, 22, 23), but little has been published about its physiology. Recently, we reported that soil and conmpost samples collected from the vicinity of Athens, Ga. contain glycolytic, anaerobic, spore-forming bacteria which grow up to 78°C (9). Thirteen strains of a Clostridium have now been isolated from these samples and from mud obtained from hot springs. This report deals with the identification of these strains as C. thermohydrosulfuricum and with the description of physiological properties of these isolates and the neotype strain E100-69 (13). A preliminary account of this study was presented at the Annual Meeting of the American Society for Microbiology, Las

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The fraction of chloroplast DNA transcribed at different stages of chloroplast development in Euglena gracilis was measured by RNA-DNA hybridization. Euglena cells were grown in the dark in a heterotrophic medium to stationary phase and then transferred to the light. Chloroplast development was monitored by the increase in the cellular chlorophyll content in the absence of cell division. Total cell RNA was isolated at various stages of chloroplast development, and hybridized in a vast excess to [125I]chloroplast DNA. The fraction of [125I]chloroplast DNA in the form of a duplex was monitored by chromatography on hydroxylapatite columns. The amount of RNA-DNA hybrid in the duplex mixture was determined by correcting for the contribution of DNA-DNA renaturation under the same conditions. The fraction of chloroplast DNA transcribed was calculated by multiplying by two the amount of single-stranded DNA in the form of an RNA-DNA hybrid. Prior to the initiation of chloroplast development (i.e., in dark grown cells) the fraction of chloroplast DNA represented as RNA transcripts in the cell is 0.53. As chloroplast development proceeds, the fraction of the chloroplast DNA transcribed decreases to 0.47. Experiments in which mixtures of various RNA samples were hybridized to the chloroplast DNA indicate that there is a small portion of chloroplast DNA transcribed at later stages of chloroplast development which is not represented as transcripts at the onset of chloroplast development. Melting properties of the RNA-DNA hybrids show that the RNA-DNA duplexes are slightly less stable than renatured [125I]chloroplast DNA.

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