Studies on Polypeptide‐Chain‐Elongation Factors from an Extreme Thermophile, Thermus thermophilus HB8

Abstract

Catalytic properties of the elongation factors from Thermus thermophilus HB8 have been studied and compared with those of the factors from Escherichia coli. 1 The formation of a ternary guanine‐nucleotide · EF‐Tu · EF‐Ts complex was demonstrated by gel filtration of the T. thermophilus EF‐Tu · EF‐Ts complex on a Sephadex G‐150 column equilibrated with guanine nucleotide. The occurrence of this type of complex has not yet been proved with the factors from E. coli. 2 The dissociation constants for the complexes of T. thermophilus EF‐Tu · EF‐Ts with GDP and GTP were 6.1 × 10⁻⁷ M and 1.9 × 10⁻⁶ M respectively. On the other hand, T. thermophilus EF‐Tu interacted with GDP and GTP with dissociation constants of 1.1 × 10⁻⁹ M and 5.8 × 10⁻⁸ M respectively. This suggests that the association of EF‐Ts with EF‐Tu lowered the affinity of EF‐Tu for GDP by a factor of about 600 and facilitated the nucleotide exchange reaction. 3 Although the T. thermophilus EF‐Tu · EF‐Ts complex hardly dissociates into EF‐Tu and EF‐Ts, a rapid exchange was observed between free EF‐Ts and the EF‐Tu · EF‐Ts complex using ³H‐labelled EF‐Ts. The exchange reaction was independent on the presence or absence of guanine nucleotides. 4 Based on the above findings, an improved reaction mechanism for the regeneration of EF‐Tu GTP from EF‐Tu · GDP is proposed. 5 Studies on the functional interchangeability of EF‐Tu and EF‐Ts between T. thermophilus and E. coli has revealed that the factors function much more efficiently in the homologous than in the heterologous combination. 6 T. thermophilus EF‐Ts could bind E. coli EF‐Tu to form an EF‐Tu (E. coli) · EF‐Ts (T. thermophilus) hybrid complex. The complex was found to exist in a dimeric form indicating that the property to form a dimer is attributable to T. thermophilus EF‐Ts. On the other hand, no stable complex between E. coli EF‐Ts and T. thermophilus EF‐Tu has been isolated. 7 The uncoupled GTPase activity of T. thermophilus EF‐G was much lower than that of E. coli EF‐G. T. thermophilus EF‐G formed a relatively stable binary EF‐G · GDP complex, which could be isolated on a nitrocellulose membrane filter. The K_d values for EF‐G · GDP and EF‐G · GTP were 6.7 × 10⁻⁷ M and 1.2 × 10⁻⁵ M respectively. The ternary T. thermophilus EF‐G · GDP · ribosome complex was again very stable and could be isolated in the absence of fusidic acid. The stability of the latter complex is probably the cause of the low uncoupled GTPase activity of T. thermophilus EF‐G.