Pentose Utilization bytheRuminal Bacterium Ruminococcus albust

Ruminococcus albus isanimportant fibrolytic ruminal bacteria whichdegrades hemicellulose andferments theresulting pentose sugars. However, little information isavailable ontheutilization ofpentoses bythis organism ortheeffect ofhexose sugars onpentose metabolism. Enzymatic studies indicated thatR.albus metabolized pentoses viathepentose phosphate pathway andpossessed constitutive transketolase activity. Cellobiose waspreferred overxylose andarabinose, anditappeared thatthedisaccharide decreased pentose metabolism byrepression oftransport activity andcatabolic enzymes (isomerases andkinases). Glucose and xylose wereco-utilized, andtransport studies suggested thatthere wasacommontransport system forboth sugars. Incontrast, glucose waspreferred overarabinose andthehexose noncompetitively inhibited the transport ofarabinose. Since R.albus lacks aglucose phosphotransferase system, theinhibition ofarabinose uptake could notbeexplained bypreviously described models ofinducer exclusion involving suchasystem. Because accumulation ofradiolabeled xylose, arabinose, andglucose proceeded intheabsence ofaproton motive force andsince transport wascorrelated withtheintracellular ATPconcentration, itappeared that monosaccharide uptake wasdriven byATPhydrolysis.