The ecdysone receptor is a hormone-dependent transcription factor that plays a central role in regulating the expression of vast networks of genes during development and reproduction in the phylum Arthropoda. The functional receptor is a heterodimer of the two nuclear receptor proteins ecdysone receptor (EcR) and ultraspiracle protein. The receptor is the target of the environmentally friendly bisacylhydrazine insecticides, which are effective against Lepidoptera but not against Hemiptera or several other insect orders. Here we present evidence indicating that much of the selectivity of the bisacylhydrazine insecticides can be studied at the level of their binding to purified ecdysone receptor ligand-binding domain (LBD) heterodimers. We report the crystal structure of the ecdysone receptor LBD heterodimer of the hemipteran Bemisia tabaci (Bt, sweet potato whitefly) in complex with the ecdysone analogue ponasterone A. Although comparison with the corresponding known LBD structure from the lepidopteran Heliothis virescens (Hv) ecdysone receptor revealed the overall mode of ponasterone A binding to be very similar in the two cases, we observed that the BtEcR ecdysteroid-binding pocket is structured differently to that of HvEcR in those parts that are not in contact with ponasterone A. We suggest that these differences in the ligand-binding pocket may provide a molecular basis for the taxonomic order selectivity of bisacylhydrazine insecticides. The nuclear receptor (NR)1 family of proteins plays a crucial role in the regulation of transcription, and its members include the receptors for steroid hormones, vitamins, thyroid hormones, and bile acids (1). The human genome contains about 48 members of this family, and these have been studied extensively as therapeutic targets (2). The Arthropoda display a more limited suite of NRs (3) about 21 of which occur in Drosophila melanogaster. Among these is the receptor for the major arthropod steroid hormone, 20-hydroxyecdysone, which is involved in the regulation of insect molting, metamorphosis, and reproduction (4 -9). The receptor is absent from mammals and is thus potentially useful as a safe insecticide target. Indeed members of the bisacylhydrazine family exert their insecticidal activity by binding to the ecdysone receptor and exhibit remarkable taxonomic order selectivity (10, 11). These compounds act selectively on the Lepidoptera and certain Coleoptera (10) but are ineffective against insects of the hemipteran order and therefore cannot be used to control certain insect pests. A study (12) of two hemipteran insect predators (Geocoris punctipes and Orius insidiosus) showed that these beneficial (predatory) hemipterans are relatively insensitive to the bisacylhydrazine tebufenozide, whereas lepidopteran insect pests are susceptible. An improved understanding of variation in the structure of the ligand-binding pockets of ecdysone receptors and the basis of the specificity of these compounds at the atomic level of detail of their interaction with the receptor may aid ...

Mature biofilm and planktonic cells of Streptococcus mutans cultured in a neutral pH environment were subjected to comparative proteome analysis. Of the 242 protein spots identified, 48 were significantly altered in their level of expression (P<0?050) or were unique to planktonic or biofilm-grown cells. Among these were four hypothetical proteins as well as proteins known to be associated with the maintenance of competence or found to possess a cin-box-like element upstream of their coding gene. Most notable among the non-responsive genes were those encoding the molecular chaperones DnaK, GroEL and GroES, which are considered to be up-regulated by sessile growth. Analysis of the rest of the proteome indicated that a number of cellular functions associated with carbon uptake and cell division were down-regulated. The data obtained were consistent with the hypothesis that a reduction in the general growth rate of mature biofilms of S. mutans in a neutral pH environment is associated with the maintenance of transformation without the concomitant stress response observed during the transient state of competence in bacterial batch cultures.

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Compared with traditional two-dimensional (2D) proteome analysis of Streptococcus mutans grown as a biofilm from a planktonic culture at steady state (Rathsam et al., Microbiol. 2005, 151, 1823-1837), the use of 2D fluorescence difference gel electrophoresis (DIGE) led to a 3-fold increase in the number of identified protein spots that were significantly altered in their level of expression (P < 0.050). Of the 73 identified proteins, only nine were up-regulated in biofilm grown cells. The results supported the previously surmised hypothesis that general metabolic functions were down-regulated in response to a reduction in growth rate in mature S. mutans biofilms. Up-regulation of competence proteins without any concomitant increase in stress-responsive proteins was confirmed, while the levels of glucosyltransferase C (GtfC), involved in glucan formation, O-acetylserine sulfhyrylase (cysteine synthetase A; CsyK), implicated in the formation of [Fe-S] clusters, and a hypothetical protein encoded by the open reading frame, SMu0188, were also up-regulated.