Primary structure of the chromosomal protein MC1 from the archaebacterium Methanosarcina sp. CHTI 55

The DNA of the thermophilic archaebacterium Methanosarcina sp. CHTI 55 has been shown to be associated with two proteins called MC1 and MC2, of molecular mass 11 kDa and 17 kDa (Chartier et al. (1988) Biochim. Biophys. Acta 951, 149-156). The most abundant of these proteins, protein MC1, can protect DNA against thermal denaturation. In the present paper we report the covalent structure of protein MC1 and its effect on transcription of DNA in vitro. The covalent structure was determined from automated sequence analysis of the protein and from structural data provided by peptides derived from cleavage of the protein at aspartic acid and arginine residues. The amino-acid sequence of protein MC1 from Methanosarcina sp. CHTI 55 is closely related to that of the protein MC1 (previously called HMb) isolated from Methanosarcina barkeri strain MS: among the nine substitutions observed between the two proteins seven are conservative. Transcription of DNA in vitro is stimulated by protein MC1 at low protein-to-DNA ratio but is inhibited at a ratio higher than 0.1 (w/w), which is the one determined in the bacterial deoxyribonucleoprotein complex.