Transcriptional linkage of Haloterax volcanii proteasomal genes with non-proteasomal gene neighbours including RNase P, MOSC domain and SAM-methyltransferase homologues

Comparative genomics reveals a common theme of 20S proteasome and proteasome-activating nucleotidase genes dispersed throughout archaeal genomes yet arranged in conserved linkages with gene homologues of translation and/or transcription machineries. To provide biological evidence for these linkages as well as insight into proteasome operon organization, transcripts of the five proteasomal genes of the halophilic archaeon Haloferax volcanii were analysed by Northern (RNA) blotting, RT-PCR and primer extension. These included psmA, psmB and psmC, encoding the 20S proteasomal subunits alpha 1, beta and alpha 2, as well as panA and panB, encoding the PanA and PanlB proteasome-activating nucleotidase proteins, respectively. All five of these genes are dispersed throughout the H. volcanii genome. For each proteasomal gene, a distinct transcript was detected by Northern blotting that was similar in size to the respective coding region. For both psmA and psmC, an additional transcript was detected that was 1.34 and 0.85 kb greater, respectively, than the coding region. Further analysis by Northern blotting and RT-PCR revealed that psmA was co-transcribed with genes encoding a Pop5 homologue of the RNase P endoRNase as well as an S-adenosylmethionine (SAM)-dependent methyltransferase. Likewise, psmC was co-transcribed with a downstream gene encoding a molybdenum cofactor sulfurase C-terminal (MOSC) domain protein. Additional proteasomal and neighbouring gene-specific transcriptional linkages were detected by RT-PCR. These results provide the first evidence that proteasome and tRNA modification genes are co-transcribed, reveal that a number of additional enzymes including those predicted to facilitate metal-sulfur cluster assembly are co-regulated with proteasomes at the transcriptional level, and provide further insight into proteasome, gene transcription in archaea.