Susan M. E. Smith

An Autoinhibitory Control Element Defines Calcium-regulated Isoforms of Nitric Oxide Synthase

Nitric oxide synthases (NOSs) are classified functionally, based on whether calmodulin binding is Ca2+-dependent (cNOS) or Ca2+-independent (iNOS). This key dichotomy has not been defined at the molecular level. Here we show that cNOS isoforms contain a unique polypeptide insert in their FMN binding domains which is not shared with iNOS or other related flavoproteins. Previously identified autoinhibitory domains in calmodulin-regulated enzymes raise the possibility that the polypeptide insert is the autoinhibitory domain of cNOSs. Consistent with this possibility, three-dimensional molecular modeling suggested that the insert originates from a site immediately adjacent to the calmodulin binding sequence. Synthetic peptides derived from the 45-amino acid insert of endothelial NOS were found to potently inhibit binding of calmodulin and activation of cNOS isoforms. This inhibition was associated with peptide binding to NOS, rather than free calmodulin, and inhibition could be reversed by increasing calmodulin concentration. In contrast, insert-derived peptides did not interfere with the arginine site of cNOS, as assessed from [3H]N G-nitro-l-arginine binding, nor did they potently effect iNOS activity. Limited proteolysis studies showed that calmodulin’s ability to gate electron flow through cNOSs is associated with displacement of the insert polypeptide; this is the first specific calmodulin-induced change in NOS conformation to be identified. Together, our findings strongly suggest that the insert is an autoinhibitory control element, docking with a site on cNOSs which impedes calmodulin binding and enzymatic activation. The autoinhibitory control element molecularly defines cNOSs and offers a unique target for developing novel NOS activators and inhibitors.

Developmental states associated with the floral transition

Floral initiation can be analyzed from a developmental perspective by focusing upon how developmental fates are imprinted, remembered, and expressed. This is not an altogether new perspective, since people studying flowering have been concerned for a long time with the commitment of meristems to form flowers and the morphological, cellular, and molecular changes associated with this commitment. What is novel is the emphasis on developmental states as opposed to physiological processes. This developmental focus indicates that there appear to be at least three major developmental states that are acquired and expressed in the process of a meristem initiating floral morphogenesis. The meristem cells must first become competent to respond to a developmental signal that evokes them into a florally determined state. The leaves are the usual source of this signal and a specific leaf may or may not have the capacity to be inductively active. When a leaf does develop the capacity for inductive activity, this capacity is usually correlated with the ontogeny of the leaf. Inductive activity, however, may be continually expressed as in some day-neutral plants or may be latent as in plants where the photoperiod is the external cue for activity. Competent shoot apical meristems respond to inductive leaf signal by being evoked into a florally determined state. Under permissive conditions this florally determined state is expressed as the initiation of floral morphogenesis. Many mechanisms have evolved to regulate entry into and expression of these developmental states. As we learn more about the developmental states associated with flowering and how they are acquired and expressed, we will understand better how the various patterns of flowering are related to one another as well as which developmental processes are common to all angiosperms.

A single-stage polymerase-based protocol for the introduction of deletions and insertions without subcloning

A single-stage polymerase-based procedure is described that allows extensive modifications of DNA. The version described here uses the QuikChange Site-Directed Mutagenesis System kit supplied by Stratagene. The original protocol is replaced by a single-stage method in which linear production of complementary strands is accomplished in separate single primer reactions. This has proved effective in introducting insertions and deletions into large gene/vector combinations without subcloning.

INSULT: a novel mutagenesis method generating high yields of closed circular mutant DNA with one primer per mutant.

INSULT, a novel method for the creation of insertions, deletions, and point mutations without subcloning, requires only one new primer per mutant, and produces circular plasmids, obviating the need for special ultracompetent cells. The method includes cycles of linear amplification with a thermophilic polymerase, and nick repair after each cycle with a thermophilic ligase. After production of multiple single-stranded copies of circular mutation-bearing plasmid DNA, addition of a generic primer followed by one or more polymerase reaction cycles generates double-stranded circular DNA bearing the desired mutation.

The maryland mammoth allele and rooting both perturb the fate of florally determined apices in Nicotiana tabacum

The stability of the florally determined state in terminal and axillary buds of two tobacco cultivars was studied. We used Hicks and Hicks Maryland Mammoth, near-isogenic cultivars of Nicotiana tabacum differing at the recessive maryland mammoth locus which confers short-day behavior. The experimental design consisted of growing plants in short-day conditions and subjecting them to three bioassays in long-day conditions: in vitro culture of apices consisting of meristems and three to four leaf primordia; rooting of buds consisting of meristems and 8 to 12 leaves, leaf primordia, and internodes; and release from apical dominance of axillary buds in situ. Cultured terminal and axillary apices expressed floral determination, indicating that meristems can be florally determined. Two lines of evidence indicate that rooting destabilizes an already acquired florally determined state: cultured apices from both axillary and terminal buds produced fewer nodes after excision than homologous buds which were rooted; and a lower percentage of rooted axillary buds from Hicks Maryland Mammoth plants expressed floral determination than did homologous axillary buds grown out in situ in noninductive conditions. Rooted buds from the two genotypes expressed floral determination at different frequencies, but produced abnormal inflorescences at similar frequencies, indicating that roots and the maryland mammoth allele influence common as well as unique processes associated with floral determination.

Competition and binding of arginine, imidazole, and aminoguanidine to endothelial nitric oxide synthase: aminoguanidine is a poor model for substrate, intermediate, and arginine analog inhibitor binding

The oxygenase domains of nitric oxide synthases are unusual in that they contain at least three ligand binding sites; these correspond to the axial heme ligand position, the substrate binding site, and the pterin binding site. Ligands can occupy portions of a site or extend into regions of adjacent sites. Depending on the size, shape, and binding mode of ligands to these positions, cooperative and anticooperative interactions mediated conformationally and by binding domain overlap can be observed. In the present study we describe competition between arginine and imidazole at the axial heme ligand position; a second imidazole, which occupies part of the arginine site in some crystal structures, is too weak to contribute to the equilibria. All spectroscopic titrations using imidazole competition depend on displacement of the heme axial imidazole ligand, which drives the ferriheme low spin. Aminoguanidine, a partial arginine analog, has multiple binding modes. It is somewhat competitive with arginine; a ternary complex forms, but the K(d) for arginine increases from 1 to 15 microM in the presence of saturating aminoguanidine. Aminoguanidine competition with imidazole is very weak, amounting to approximately a factor of two increase in K(d). This implies that aminoguanidine has multiple binding modes and is not well described as an arginine analog. The major binding mode occupies part of the binding site but does not extend into the imidazole axial ligand binding domain and probably corresponds to the crystal structure. The other binding mode is not significantly overlapped with the arginine site.

Nitric Oxide Synthases: Structure, Function, and Control

Nitric oxide synthases are large modular enzymes that produce NO and citrulline from arginine at the expense of NADPH and O2. We have modeled the reductase region, which corresponds roughly to the C-terminal half of the molecule, using the available crystal structures of flavodoxin (FMN-binding domain) and ferredoxin NADPH reductase (FAD- and NADPH-binding domains). This has enabled us to identify important sequence regions that interact with cofactors and to show that the N-terminal boundary of the reductase domain extends to within a few residues of the CaM-binding site. A large (40–50 residues) insertion in the FMN-binding domain of cNOS, located -80 residues downstream from the CaM site, is the major sequence difference between iNOS and cNOS. This insertion is directly adjacent to the CaMbinding site on the three-dimensional structure. Several lines of evidence suggest that it functions as a control element.

The Maryland mammoth allele reduces floral stimulus activity in stem piece explants of Nicotiana tabacum (Solanaceae).

The response of axillary buds to floral stimulus activity in stem pieces was examined in two near-isogenic cultivars of tobacco that differ in the recessive maryland mammoth (mm) allele, which confers short-day behavior. All axillary buds from day-neutral plants assayed on six-internode stem pieces made few nodes (less than 20) before flowering, while axillary buds from plants homozygous for mm assayed on six-internode stem pieces either did not flower in noninductive conditions or made many nodes before flowering in inductive conditions. About 80% of day-neutral axillary buds grafted onto day-neutral stem pieces did not respond to floral stimulus in stem pieces, indicating that the floral stimulus in stem pieces is ephemeral. In other graft combinations, the proportion of axillary buds that did respond to floral stimulus in stem pieces was substantially reduced from the 20% of day-neutral buds on day-neutral stem pieces that responded. These results indicate that the mm allele probably reduces both the amount of floral stimulus activity in stem pieces and the competence of axillary buds to respond.