Some articles on proteobacterium:

Eukaryota - Origin of Eukaryotes - Hypotheses For The Origin of Eukaryotes - Chimeric Models
... linkage of an anaerobic and autotrophic methanogenic archaeon (host) with an alpha-proteobacterium (the symbiont) gave rise to the eukaryotes ... based on metabolic symbiosis (syntrophy) between a methanogenic archaeon and a delta-proteobacterium ... In earlier stages, an alpha-proteobacterium became a member of this integration, and later developed into the mitochondrion ...