Corynebacterium glutamicum is a biotin-auxotrophic bacterium and some strains efficiently produce glutamic acid under biotin-limiting conditions. In an effort to understand C. glutamicum metabolism under biotin limitation, growth of the type strain ATCC 13032 was investigated in batch cultures and a time-course analysis was performed. A transient excretion of organic acids was observed and we focused our attention on lactate synthesis. Lactate synthesis was due to the ldh-encoded l-lactate dehydrogenase (Ldh). Features of Ldh activity and ldh transcription were analysed. The ldh gene was shown to be regulated at the transcriptional level by SugR, a pleiotropic transcriptional repressor also acting on most phosphotransferase system (PTS) genes. Electrophoretic mobility shift assays (EMSAs) and site-directed mutagenesis allowed the identification of the SugR-binding site. Effector studies using EMSAs and analysis of ldh expression in a ptsF mutant revealed fructose 1-phosphate as a highly efficient negative effector of SugR. Fructose 1,6-bisphosphate also affected SugR binding.
Photoheterotrophic growth of cell suspensions of Nicotiana tabacum L. (cv. Xanthi) in organic culture medium enriched in sucrose (30 g per liter) showed a classical sigmoid growth curve. The cells developed functional chloroplast structures during the exponential growth phase, when their chlorophyll content increased steadily. A limited drop (30%) in the chlorophyll amount and structural changes of the plastids (starch accumulation) were observed during the lag phase. The measurements of photosynthetic capacities (O2 evolution and CO2 fixation) during the growth cycle revealed changes in the photosynthetic ratio (O2/CO2), which was near 1 during the lag and stationary phases and near 2 during exponential growth. During exponential growth there was also a rapid NO3− uptake. Analysis of label distribution among the products of 14CO2 fixation showed that both CO2 assimilation pathways, linked to the ribulose‐biphosphate carboxylase (the autotrophic pathway) and to phosphoenolpyruvate carboxylase (the non‐autotrophic pathway) were operative with an important increase of the capacity of the latter during the exponential growth phase. Maximum rate of oxygen evolution, either endogenous or with p‐benzoquinone as Hill reagent, as well as the increased CO2 Fixation capacity via the non‐autotrophic pathway during the exponential phase were concomitant with a high cyanide inhibited O2 uptake.
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