CATABOLITE REGULATION IN A DIAUXIC STRAIN AND A NONDIAUXIC STRAIN OF STREPTOCOCCUS-BOVIS

Streptococcus bovis JB1 utilized glucose preferentially to lactose and grew diauxically, but S. bovis 581AXY2 grew nondiauxically and used g lucose preferentially only when the glucose concentration was very hig h (greater than 5 mM). As little as 0.1 mM glucose completely inhibite d the lactose transport of JB1. The lactose transport system of 581AXY 2 was at least tenfold less sensitive to glucose, and 1 mM glucose cau sed only a 50% inhibition of lactose transport. Both strains had phosp hotransferase systems (PTSs) for glucose and lactose. The glucose PTSs were constitutive, but little lactose PTS activity was detected unles s lactose was the energy source for growth. JB1 had approximately thre efold more glucose PTS activity than 581AXY2 (1600 versus 600 nmol glu cose (mg protein)(-1)(min)(-1). The glucose PTS of JB1 showed normal M ichaelis Menten kinetics, and the affinity constant (K-s) was 0.12 mM. The glucose PTS of 581AXY2 was atypical, and the plot of velocity ver sus velocity/substrate was biphasic, The low capacity system had a K-s of 0.20 mM, but the K-s of the high capacity system was greater than 6 mM. On the basis of these results, diauxic growth is dependent on th e affinity of glucose enzyme II and the velocity of glucose transport.