Effects of L-glutamine and HS compounds on growth and sporulation metabolism of Achlya

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Abstract
A study of [32P]orthophosphate labeling of purine and pyrimidine nucleotides and dinucleoside polyphosphates, HS3, HS2 and HS1, during ontogeny of the fungus Achlya showed that HS compounds were labeled very quickly at late stages of growth and then declined to low levels just as the overall growth decreased. At this time, sporangial formation was initiated and HS3 increased again. By contrast, the common cellular nucleotides stayed at a low constant level. When 1 mM L-glutamine (D-glutamine was ineffective) was added to the growth medium at this time, sporangial formation was aborted. When cells growing in a rich medium of peptone, yeast extract and glucose were transferred to a nutrient-deficient medium of 0.1 mM CaCl2, they developed sporangia within 8 h and sporulated by 10 h. In this period, the cellular levels of HS compounds increased many-fold (HS3, 6-fold; HS2, 18-fold; and HS1, 55-fold). In the presence of 1 mM L-glutamine, cellular HS decreased to barely perceptible levels and sporulation was aborted. Several antimetabolites that block de novo biosynthesis of nucleotides, diazouracil, azaserine and methotrexate, stimulated further HS accumulation in nutrient-deficient medium but did not block sporulation. Uridine, guanosine and adenosine caused a partial reduction of HS compounds and correspondingly delayed sporulation. Thymidine was ineffective. Actinomycin D (5 .mu.g/ml) caused HS2 and HS1 to accumulate many-fold and inhibited sporulation. Cycloheximide (10 .mu.g/ml) caused HS3 to increase several-fold and also blocked sporulation. In nutrient-deficient medium, it seemed necessary that all 3 HS compounds increase simultaneously for sporulation to occur. HS compounds proved to be powerful inhibitors of enzymes involved with nucleic-acid metabolism. They inhibited DNA-dependent RNA polymerases with HS2 being more effective than HS3; HS1 inhibited DNA polymerase specifically. This is probably meaningful as low intracellular levels of HS compounds were detected when nucleic-acid synthesis was proceeding optimally. The uracil moiety of HS compounds was labeled with [14C]aspartate, CO2 and orotate, not uracil or uridine. Neither [14C]adenine nor [14C]adenosine was incorporated into HS3. HS compounds may be biosynthesized by the de novo pathways of nucleotide biosynthesis. A role for HS compounds as sensors of N insufficiency in Achlya as a prelude to sporulation is suggested.