Cryptic biodiversity in the cytogenome of bird-biting blackflies in North Africa.

Bird-biting blackflies in the Simulium (Eusimulium) aureum group (Diptera: Simuliidae) are widespread vectors of Leucocytozoon and Trypanosoma parasites. The polytene chromosomes of 619 larvae of the three nominal members of the S. aureum group in North Africa were evaluated cytogenetically for cryptic biodiversity. Seven chromosomal segregates were discovered among 29 populations in Algeria and Morocco. This diversity was based primarily on two chromosomal inversions, which have assumed unique roles in different lineages, including sex linkage, fixation, loss and autosomal polymorphism. Reproductive isolation was demonstrated for six of the seven segregates, doubling the number of species known in the area. Four species were linked with existing names: (a) Simulium mellah Giudicelli & Bouzidi, which is known only from North African high-salinity habitats; (b) Simulium petricolum (Rivosecchi), which is tentatively conspecific with continental European populations; (c) Simulium rubzovianum (Sherban) and its synonym Simulium latinum (Rubtsov), which is widely distributed from North Africa across Europe into Western Asia, and (d) Simulium velutinum (Santos Abreu) and its new synonym Simulium tenerificum Crosskey, which is restricted to North Africa and the Canary Islands. Of the remaining entities, two are new species precinctive to North Africa and one, known only from Morocco, is of undetermined taxonomic status.

A hypothetical model of the foreign antigen binding site of class II histocompatibility molecules.

Class II and class I histocompatibility molecules allow T cells to recognize 'processed' polypeptide antigens. The two polypeptide chains of class II molecules, alpha and beta, are each composed of two domains (for review see ref. 6); the N-terminal domains of each, alpha 1 and beta 1, are highly polymorphic and appear responsible for binding peptides at what appears to be a single site and for being recognized by MHC-restricted antigen-specific T cells. Recently, the three-dimensional structure of the foreign antigen binding site of a class I histocompatibility antigen has been described. Because a crystal structure of a class II molecule is not available, we have sought evidence in class II molecules for the structural features observed in the class I binding site by comparing the patterns of conserved and polymorphic residues of twenty-six class I and fifty-four class II amino acid sequences. The hypothetical class II foreign-antigen binding site we present is consistent with mutation experiments and provides a structural framework for proposing peptide binding models to help understand recent peptide binding data.

The foreign antigen binding site and T cell recognition regions of class I histocompatibility antigens.

Most of the polymorphic amino acids of the class I histocompatibility antigen, HLA-A2, are clustered on top of the molecule in a large groove identified as the recognition site for processed foreign antigens. Many residues critical for T-cell recognition of HLA are located in this site, in positions allowing them to serve as ligands to processed antigens. These findings have implications for how the products of the major histocompatibility complex (MHC) recognize foreign antigens.

Structure of the human class I histocompatibility antigen, HLA-A2.

The class I histocompatibility antigen from human cell membranes has two structural motifs: the membrane-proximal end of the glycoprotein contains two domains with immunoglobulin-folds that are paired in a novel manner, and the region distal from the membrane is a platform of eight antiparallel beta-strands topped by alpha-helices. A large groove between the alpha-helices provides a binding site for processed foreign antigens. An unknown 'antigen' is found in this site in crystals of purified HLA-A2.

Tertiary structural similarity between a class A beta-lactamase and a penicillin-sensitive D-alanyl carboxypeptidase-transpeptidase.

beta-Lactam antibiotics--the penicillins, cephalosporins and related compounds--act by inhibiting enzymes that catalyse the final stages of the synthesis of bacterial cell walls. Recent crystallographic studies of representative enzymes are beginning to reveal the structural bases of antibiotic specificity and mechanism of action, while intensive efforts are being made to understand the beta-lactamase enzymes that are largely responsible for bacterial resistance to these antibiotics. It has been suggested that the beta-lactamases and beta-lactam target enzymes may be evolutionarily related and some similarity of amino-acid sequence around a common active-site serine residue supports this idea. We present here the first evidence from a comparison of three-dimensional structures in support of this hypothesis: the structure of beta-lactamase I from Bacillus cereus is similar to that of the penicillin-sensitive D-alanyl-D-alanine carboxypeptidase-transpeptidase from Streptomyces R61.