THE ARCHAEAL DNAK-DNAJ GENE-CLUSTER - ORGANIZATION AND EXPRESSION IN THE METHANOGEN METHANOSARCINA-MAZEI

The organization and expression of the first archaeal dnaK-dnaJ gene cluster cloned and sequenced have been elucidated. The work focused on the methanogen Methanosarcina mazei strain S-6, but a survey of two other strains (JC3 and LYC) and species (Methanosarcina sp. JCV and Methanosarcina acetivorans) showed that the findings are pertinent to other mesophilic methanosarcinas as well. The organization and some expression features of the archaeal genes resemble eubacterial equivalents for which comparable sequence information is available. However, the archaeal genes also display characteristics that are distinct from those of eubacterial and eucaryotic homologs. dnaK and dnaJ are transcribed into monocistronic messages. The initiation site is the same for transcription under optimal cell-growth conditions, and under stress due to a temperature upshift. The two genes are expressed constitutively at lower levels than those observed after heat shock. The constitutive and post-heat-shock expression levels are higher for dnaK than for dnaJ. Both genes withstand heat shocks of at least one and a half hours without a decline in transcript levels. While the transcription termination signals are to some extent reminiscent of those of eubacteria, the initiation signals are not. These have archaeal characteristics, which resemble those of eukaryotes. The intergenic dnaK-dnaJ region contains inverted repeats. These have the potential to build firm stem-loops in the transcript and in single-stranded DNA.