Agaricus bisporus, grown under standard composting conditions, was evaluated for its ability to produce lignin-degrading peroxidases, which have been shown to have an integral role in lignin degradation by wood-rotting fungi. The activity of manganese peroxidase was monitored throughout the production cycle of the fungus, from the time of colonization of the compost through the development of fruit bodies. Characterization of the enzyme was done with a crude compost extract. Manganese peroxidase was found to have a pl of 3.5 and a pH optimum of 5.4 to 5.5, with maximal activity during the initial stages of fruiting (pin stage). The activity declined considerably with fruit body maturation (first break). This apparent developmentally regulated pattern parallels that observed for laccase activity and for degradation of radiolabeled lignin and synthetic lignins by A. bisporus. Lignin peroxidase activity was not detected in the compost extracts. The correlation between the activities of manganese peroxidase and laccase and the degradation of lignin in A. bisporus suggests significant roles for these two enzymes in lignin degradation by this fungus.

SummaryPhase-variable expression of type 1 fimbriae is, in part, controlled by site-specific DNA inversion of the fim switch in Escherichia coli. Of the two fim recombinases (FimB and FimE) that catalyse the inversion reaction, FimE exhibits a strong bias for phase switching from the ON to the OFF orientation. The specificity associated with fimE is the result of two different mechanisms: (i) FimE exhibits a preference for the invertible element in the ON orientation as substrate for recombination; (ii) the invertible element in the OFF orientation acts in cis to inhibit recombinase activity (orientational control). We show here that the invertible element negatively regulates fimE, even though expression of a fimElacZYA transcriptional fusion is unaffected by orientational control. The fimE transcript extends into the invertible region and hence switch ON-specific and switch OFF-specific mRNA contain different sequences. Furthermore, we show that orientational control is suppressed by the insertion of a structured RNA (tRNA Gly ) between fimE and the fim switch, indicating that the switch OFF-specific mRNA is inactivated by 3 0 to 5 0 degradation. Analysis of the fim switch reveals that it contains two inhibitory elements that exert orientational control independently.

Etiolated seedlings of cucumber, muskmelon, watermelon, pumpkin, and squash were inoculated with Colletotrichum lagenarium (Pass.) Ell. & Halst. race 1, Colletotrichum atramentarium (Berk. & Br.) Taub., or Helminthosporium carbonum Ull. The fungi germinated and produced appressoria equally well on all hosts. Penetrations into host cells, however, were routinely observed only in the interactions of C. lagenarium with cucumber, muskmelon, and watermelon. Histochemical staining of the other host–pathogen interactions (nonhost interactions) revealed that the lack of penetration by fungi into nonhost tissue was associated with the deposition of lignin in the upper and lateral host epidermal cell walls around appressoria. Treatment of the lignified tissues with 0.5 M NaOH or hot ethanol did not alter staining reactions. Treatment with hot aminoethanol, a delignifying agent, did eliminate stainings. Cupric oxide oxidation of lignified epidermal cell walls from each host revealed that each host produced a p-coumaryl-rich, guaiacyl-poor ligninlike material. These results suggest that lignification may be a general nonhost resistance response in the Cucurbitaceae.

, lllinois 61 604 (T.H.)The gene encoding trichodiene synthase (Trit;), a sesquiterpene synthase from the fungus Fusarium sporofrichioides, was used t o transform tobacco (Nicotiana tabacum). Trichodiene was the sole sesquiterpene synthase product in enzyme reaction mixtures derived from unelicited transformant cell-suspension cultures, and both trichodiene and 5-epi-aristolochene were observed as reaction products following elicitor treatment. lmmunoblot analysis of protein extracts revealed the presence of trichodiene synthase only in transformant cell lines producing trichodiene. In vivo labeling with [3Hlmevalonate revealed the presence of a novel trichodiene metabolite, 1 5-hydroxytrichodiene, that accumulated in the transformant cell-suspension cultures. I n a trichodiene-producing transformant, the leve1 of 1 5-hydroxytrichodiene accumulation increased after elicitor treatment. In vivo labeling with [14C1acetate showed that the biosynthetic rate of trichodiene and 15-hydroxytrichodiene also increased after elicitor treatment. lncorporation of radioactivity from [14C]acetate into capsidiol was reduced following elicitor treatment of a trichodiene-producing transformant as compared with wild type. These results demonstrate that sesquiterpenoid accumulation resulting from the constitutive expression of a foreign sesquiterpene synthase is responsive t o elicitation and that the farnesyl pyrophosphate present in elicited cells can be utilized by a foreign sesquiterpene synthase t o produce high levels of novel sesquiterpenoids.Sesquiterpenoids are thought to be prominent in plantfungal interactions. Members of this structurally diverse group of cyclic terpenoids appear to function both as defensive compounds in plants and as virulence factors in fungal plant pathogens. The best characterized examples of this include the accumulation of antimicrobial sesquiterpenoids in tobacco (Nicotiana tabacum) and other Solanaceae following exposure to fungi or fungal cell-wall fragments (Vogeli and Chappell, 1988; Zook and Kuc, 1991) and the production of phytotoxic sesquiterpenoids by fungal plant pathogens that enhance their virulence on certain plant hosts (Proctor et al., 1995). Sesquiterpenoids also frequently occur as components of plant essential oils and contribute to the flavor and fragrance of some agricultura1 products (Templeton, 1969).

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