Nourseothricins (syn. Streptothricins), a group of nucleoside peptides produced by several streptomycete strains, contain a poly b-lysine chain of variable length attached in amide linkage to the amino sugar moiety gulosamine of the nucleoside portion. We show that the nourseothricin-producing Streptomyces noursei contains an enzyme (NpsA) of an apparent M r 56 000 that speci®cally activates b-lysine by adenylation but does not bind to it as a thioester. Cloning and sequencing of npsA from S. noursei including its¯ank-ing DNA regions revealed that it is closely linked to the nourseothricin resistance gene nat1 and some other genes on the chromosome possibly involved in nourseothricin biosynthesis. The deduced amino-acid sequence revealed that NpsA is a stand-alone adenylation domain with similarity to the adenylation domains of nonribosomal peptide synthetases (NRPS). Further analysis revealed that S. noursei contains a b-lysine binding enzyme (NpsB) of about M r 64 100 which can be loaded by NpsA with b-lysine as a thioester. Analysis of the deduced amino-acid sequence from the gene (npsB) of NpsB showed that it consists of two domains. The N-terminal domain of 100 amino-acid residues has high similarity to PCP domains of NRPSs whereas the 450-amino-acid C-terminal domain has a high similarity to epimerization (E)-domains of NRPSs. Remarkably, in this E-domain the conserved H-H-motif is changed to H-Q, which suggests that either the domain is nonfunctional or has a specialized function. The presence of one single adenylating b-lysine activating enzyme in nourseothricin-producing streptomycete and a separate binding protein suggests an iteratively operating NRPS-module catalyses synthesis of the poly b-lysine chain.

Ileal bile acid-binding protein (I-BABP) is a 14 kDa cytosolic protein which binds bile acids with a high a⁄nity. It is thought to be implicated in the enterohepatic circulation of bile acids and, hence, in cholesterol homeostasis. Using a combination of in vivo and in vitro experiments, we have recently shown that I-BABP gene expression can be indirectly up-regulated by cholesterol through the activation of sterol-responsive elementbinding protein 1c (SREBP1c) by liver X-receptor (LXR). We report here that I-BABP can be also a direct target for LXR. I-BABP regulation by LXR is maintained when the SREBP binding site is deleted in the I-BABP promoter and occurs, in the absence of conventional LXRE sequences, through an IR1 sequence previously identi¢ed as a farnesoid X-receptor-responsive element (FXRE). Electrophoretic mobility shift assays demonstrated that the LXR/RXR heterodimer speci¢cally recognizes the FXRE. Collectively, these data strongly suggest that LXR can regulate the I-BABP gene by both direct and indirect mechanisms.

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