The Sac10b Homolog in Methanococcus maripaludis Binds DNA at Specific Sites

The Sac10b protein family, also known as Alba, is widely distributed in Archaea. Sac10b homologs in thermophilic Sulfolobus species are very abundant. They bind both DNA and RNA with high affinity and without sequence specificity, and their physiological functions are still not fully understood. Mma10b from the euryarchaeote Methanococcus maripaludis is a mesophilic member of the Sac10b family. Mma10b is not abundant and constitutes only approximately 0.01% of the total cellular protein. Disruption of mma10b resulted in poor growth of the mutant in minimal medium at near the optimal growth temperature but had no detectable effect on growth in rich medium. Quantitative proteomics, real time reverse transcription-PCR, and enzyme assays revealed that the expression levels of some genes involved in CO(2) assimilation and other activities were changed in the Deltamma10b mutant. Chromatin immunoprecipitation suggested a direct association of Mma10b with an 18-bp DNA binding motif in vivo. Electrophoretic mobility shift assays and DNase I footprinting confirmed that Mma10b preferentially binds specific sequences of DNA with an apparent Kd in the 100 nM range. These results suggested that the physiological role of Mma10b in the mesophilic methanococci is greatly diverged from that of homologs in thermophiles.