Stability of Protein and Ribonucleic Acid in Bacillus stearothermophilus

Abstract

The turnover of protein in a prototrophic strain of Bacillus stearothermophilus during exponential growth in a salts medium with glucose or succinate as carbon source was about 4 %/h and in a richer nutrient broth medium about 23 %/h. Protein degradation under non-growing conditions conformed to a similar pattern. The turnover of RNA (non-messenger) was about I %/h in salts medium and about g %/h in nutrient broth. The turnover of protein and R N A in the thermophile is thus moderate rather than massive. This conclusion was confirmed by measurement of the decay of a specific enzyme, isocitrate lyase, in the prototroph and of the overall protein turnover in a non-prototrophic strain of B. stearothermophilus. The half-lives of a number of enzyme systems in intact cells of the prototrophic thermophile at its optimum growth temperature showed some variation but indicated a significant rate of inactivation. Such decay of protein in vivo apparently accounts for the moderate protein turnover observed during growth. I N T R O D U C T I O NThe rate of turnover of proteins, and also of nucleic acids (other than mRNA), is very low (less than I %/h) in mesophilic bacteria during exponential growth and somewhat higher (up to 5 %/h) under non-growing conditions (Mandelstam, 1960; Halvorson, 1962). Protein turnover in the absence of growth appears to occur in these organisms in response to conditions in which synthesis of new proteins takes place in nutritionally deprived environments, such as in sporulation (Mandelstam & Waites, 1968) and the intermediate lag phase of diauxic growth (Halvorson, 1962). A different function for protein turnover in thermophilic micro-organisms was envisaged by Allen's (1 953) hypothesis that thermophiles compensate for the breakdown of macromolecules, particularly proteins, at their high growth temperatures by rapid resynthesis. Although this theory has now been overshadowed by the finding that thermophile proteins are invariably more thermostable than their mesophilic counterparts, there is apparently no agreement on the question of the extent of protein turnover in thermophilic organisms (Goldberg & Dice, 1974). Thus Bubela & Holdsworth (1966) reported exceedingly rapid protein turnover in Bacillus stearothermophilus, suggesting that this result lent support to Allen's idea; Epstein & Grossowicz (1969b), on the other hand, observed that the rate of protein turnover in their prototrophic strain of B. stearothermophilus was similar to that found in mesophiles.We examined the degradation of macromolecules, particularly proteins, in B. stearo-* Present address :

Isolation and characterization of isocitrate lyase from a thermophilic Bacillus sp

“…6 and 15h respectively. The stability of the enzyme in the pure state is considerably greater than in cell-free extracts (Griffiths & Sundaram, 1973), but more similar to that in the intact cell (Coultate et al, 1975). At higher temperatures KCI has little effect on the thermostability of the pure enzyme.…”