No abstract

Decarboxylation of ferulate to 4-vinylguaiacol is associated with the production of a phenolic off-flavour in beer. The ability (Pof+) of non-brewing strains of Saccharomyces cereuisiae to carry out this reaction has been assigned, by tetrad analysis, to a single nuclear gene. This gene (POFI) is dominant in heterozygous diploids and segregates independently from MATa/MA Ta, lys2 and DEXI. Therefore, elimination of the Pof+ phenotype from strains intended for brewing is feasible by either mutation or genetic segregation. Since ferulate was decarboxylated by cell-free supernatants derived from Pof+ strains, but not by similar fractions from Pof-strains, POFI encodes production of a cellular decarboxylase. Strains carrying POFI also decarboxylated coumarate and cinnamate in vivo, but with caffeate, 4-hydroxybenzoate, 4-hydroxyphenylacetate, styrylacetate, mandelate or phenylpropionate as substrates, the corresponding decarboxylation products were not detected. POFl may confer resistance to inhibitory effects of naturally occurring cinnamates on yeast growth.

SUMMARYSulphate-limited continuous cultures of Klebsiella pneumoniae showed a proportional repression of nitrogenase activity with increasing concentrations of ammonium ion in the influent medium. A fully repressed population had a 50 % greater bacterial density than a fully derepressed one. On derepression, synthesis of nitrogenase lagged for 90 min after exhaustion of NH,+ from the medium but was complete within one doubling time. Casamino acids did not decrease the prefixation lag obtained under sulphate-limited conditions in the chemostat. Longer lags were obtained when NH,+ was exhausted under N-limited conditions. Such lags were decreased by Casamino acids, yeast extract, or L-aspartate. Aspartate-N completely repressed nitrogenase under sulphate-or carbon-limited conditions but not under nitrogen limitation. In the initial stages of repression of nitrogenase synthesis by NH,+, apparent production of active enzyme continued for a short time in the absence of protein synthesis de novo; nitrogenase was then diluted out as the organisms multiplied. Repression by NH,+ was at the level of mRNA transcription according to studies with rifampicin and chloramphenicol. ' Coding capacity' for nitrogenase synthesis declined with a half-life of about 4.5 min following inhibition of RNA synthesis with rifampicin. NH,+ did not influence the decay rate but stimulated translation of such nitrogenase-specifying mRNA as had been initiated.

5: U M M .A R YIncreasing concentrations of animoni um ions in the medium of nitrogen-fixing, sulphate-limited continuous cultures of Azotobacter chroococcum caused a proportionate repression of nitrogenase activity; free NH,+ could be detected in the extracellular culture fluid only when nitrogenase activity was wholly repressed.The NH,T concentrations giving 50';/(, or IOO ( ; k repression were proportional to the population density. Nitrate ions repressed with similar stoichiometry ; glutamate, glutanline and aspartate did not repress and were not metabolized; repressed and derepressed populations contained equal amounts and proportions of glutamateforming enzymes. Repressed populations lacked both enzymatic components of nitrogenase. The intracellular free amino acid pools were typical of Gram-negative bacteria; an increase in the degree of repression was associated with an increase in the pool levels of ammonia, aspartate and glutamate. Nitrogen-fixing populations possessed a convoluted intracytoplasmic membrane system which was absent from ammonia-assimilating organisms, but the phospholipid contents of the two types of population were similar. All members of a half-repressed population possessed these membranes, but to a lesser extent that fully derepressed populations.When N,-fixing chemostat populations were abruptly exposed to repressive concentrations of ammonium succinate, repression occurred exponentially and nitrogenase activity disappeared from the culture faster than wash-out of stable enzyme. Repression was not alleviated by exogenous cyclic AMP. Derepression was complete, according to the acetylene test, within half a doubling time of disappearance of free ammonium ions from the culture.

149 strains of bacteria, mostly brewery contaminants able to spoil wort or beer, and 12 brewing strains of yeast (8 ale and 4 lager strains) have been screened using a well-test assay for sensitivity to the food preservative, Nisin (E234), Nisin inhibited growth of 92% of the gram-positive strains, predominantly lactic acid bacteria of the genera Lactobacillus and Pediococcus. In contrast, all 32 gram-negative strains tested, except 3 Flavobacter strains, were Nisin-resistant; in addition none of the brewing yeasts showed Nisin-sensitivity. Therefore. Nisin has potential applications in preventing spoilage of worts or beers by lactic acid bacteria.