Clive N.A. Trotman

Physiological and Biochemical Aspects of Artemia Ecology

This chapter considers life cycle-dependent biochemical and physiological adaptations critical to the survival of Artemia in nature. Thus, the encysted gastrula embryo (‘cyst’) is arguably the most resistant of all animal life history stages to extremes of environmental stress, while the motile stages are among the best osmoregulators in the animal kingdom. These and other adaptations are emphasized not only because they are essential to the success of brine shrimp, enabling them to cope with the many challenges presented by their harsh environment, but because this animal is a useful model system for the study of a variety of general biological processes. We hope our coverage will encourage others to study this remarkable organism.

Evolutionary history of introns in a multidomain globin gene.

TheArtemia hemoglobin contains two sub-units that are similar or different chains of nine globin domains. The domains are ancestrally related and are presumed to be derived from copies of an original single-domain parent gene. Since the gene copies have remained in the same environment for several hundred million years they provide an excellent model for the investigation of intron stability.The cDNA for one of the two types of nine-domain subunit (domains T1–T9) has been sequenced. Comparison with the corresponding genomic DNA reveals a total of 17 intradomain introns. Fourteen of the introns are in locations on the protein that are conventional in globins of other species. In eight of the nine domains an intron corresponds to the B helix, amino acid B12, following the second nucleotide (phase 2), and in six domains a G-helix intron is located between G6 and G7 (phase 0). The consistency of this pattern is supportive of the introns having been inherited from a single-domain parent gene. The remaining three introns are in unconventional locations. Two occur in the F helix, either in amino acid F3 (phase 1) in domain T3, or between F2 and F3 (phase 0) in domain T6. The two F introns strengthen an interpretation of intron inheritance since globin F introns are rare, and in domains T3 and T6 they replace rather than supplement the conventional G introns, as though displacement from G to F occurred before that part of the gene became duplicated. It is inferred that one of the F introns subsequently moved by one nucleotide. Similarly, the third unconventional intron location is the G intron in domain T4 which is in G6, phase 2, one nucleotide earlier than the other G introns. Domain T4 is also unusual in lacking a B intron. The pattern of introns in theArtemia globin gene supports a concept of general positional stability but the exceptions, where introns have moved out of reading frame, or have moved by several codons, or have been deleted, suggest that intron displacements can occur after inheritance from an ancient source.

Unexpected intron location in non-vertebrate globin genes

The Caenorhabditis elegans and Artemia T4 globin sequences are highly homologous with other invertebrate globins. The intron/exon patterns of their genes display a single intron in the E and G helices respectively, Precoding introns in multirepeat globins are inserted in homologous positions. Comparison of the intron/exon patterns in the known globin gene sequences demonstrates that they are more diverse than first expected but nevertheless can be derived from an ancestral pattern having 3 introns and 4 exons.

Interdomain linkage in the polymeric hemoglobin molecule of Artemia

Artemia has evolved the longest known concatenation of hemoglobin domains, the α subunit containing nine domains and the β subunit having a similar size. Translation of the cDNA sequence of the α subunit reveals eight regions of inter-domain polypeptide linking together the nine heme-binding domains, together with partially analogous sequences preceding the first domain and following the last. Analysis of the structural possibilities of the linker sequences suggests how the domains may be organized in the subunit.The interdomain linker sequences were 14%–64% identical (62%–91% similar by Dayhoff substitution matrix) and approximately 14 residues in length including a consensus -Val-Asp-Pro-Val-Thr-Gly-Leu-. The linker composition resembled that of the 11 amino acid pre-A leader sequence of Petromyzon marinus (lamprey) hemoglobin V, the structure of which is known. Prediction of structure from the Artemia linker sequences indicated a nonhelical, turn-associated linker which could be modeled to the Petromyzon leader. Measurements confirmed that such a structure could support the packing of nine Artemia domains into a polymeric subunit of annular shape, two of which subunits (which can be similar or dissimilar) comprise the physiological molecule.The position of interdomain introns and the character of a variable residue early in the linker are compatible with the nine-domain polymer having evolved through gene duplication reflected in globin domain fusion incorporating an extension specifically of the N-terminus. The multiplication of an original single-domain globin gene to give the present nine is estimated from sequence differences, allowing for multiple mutations at individual sites, to have occurred in a period at least 500–700 million years ago.

Variable Substitution Rates of the 18 Domain Sequences in Artemia Hemoglobin

Abstract. The Artemia hemoglobin is a dimer comprising two nine-domain covalent polymers in quaternary association. Each polymer is encoded by a gene representing nine successive globin domains which have different sequences and are presumed to have been copied originally from a single-domain gene. Two different polymers exist as the result of a complete duplication of the nine-domain gene, allowing the formation of either homodimers or the heterodimer. The total population size of 18 domains comprising nine corresponding pairs, coupled with the probability that they reflect several hundred million years of evolution in the same lineage, provides a unique model in which the process of gene multiplication can be analyzed. The outcome has important implications for the reliability of local molecular clocks.The two polymers differ from each other at 11.7% of amino acid sites; however when corresponding individual domains are compared between polymers, amino acid substitution fluctuates by a factor of 2.7-fold from lowest to highest. This variation is not obvious at the DNA level: Domain pair identity values fluctuate by 1.3-fold. Identity values are, however, uncorrected for multiple substitutions, and both silent and nonsilent changes are pooled. Therefore, to determine the variability in relative substitution rates at the DNA level, we have used the method of Li (1993, J Mol Evol 36:96–99) to determine estimates of nonsynonymous (KA) and synonymous (KS) substitutions per site for the nine pairs of domains. As expected, the overall level of silent substitutions (KS of 56.9%) far exceeded nonsilent substitutions (KA of 6.7%); however, for corresponding domain pairs, KA fluctuates by 2.3-fold and KS by 1.7-fold. The large discrepancies reflected in the expressed protein have accrued within a single lineage and the implication is that divergence dates of different genera based on amino acid sequences, even with well-studied proteins of reasonable size, can be wrong by a factor well in excess of 2.

ANCIENT AND RECENT INTRON STABILITY IN THE ARTEMIA HEMOGLOBIN GENE

Abstract.Artemia has evolved three distinct hemoglobins formed by the association of two nine-domain globin polymers. Sequence analysis of cDNA clones corresponding to two polymers, named T and C, indicates that their genes are the products of a duplication event some 60 million years ago. The present study indicates the presence of 22 introns in each of the T and C polymer genes. The 22 introns are classified into two groups: 17 correspond to positions within globin domains, and 5 correspond to interdomain linkers (or N- and C-terminal extensions). Intron position and reading frame phase are precisely conserved between T and C polymers for all 22 introns, but within each gene the position and phase are not always consistent from domain to domain or from linker to linker. The discordance of Artemia hemoglobin introns is discussed in terms of different model mechanisms and constraints: intron sliding, intron loss or gain, and the exon definition model of primary transcript RNA splicing. The results suggest that constraints of pre-mRNA processing should be considered when considering intron positional changes in homologous genes.

Localization of the release factor-2 binding site on 70 S ribosomes by immuno-electron microscopy☆

In protein synthesis Escherichia coli release factor-2 binds to 70 S ribosomes when the termination codon UAA or UGA appears at the decoding site. The weak interaction between factor and ribosome has been stabilized in vitro by chemical cross-linking. Factor so bound can still be recognized by a specific antibody to release factor-2. Examination of the resulting immuno-complexes by electron microscopy revealed 70 S ribosomes in different projection forms, and the occasional dissociated subunit labelled with antibody. The antibody-binding site was localized on previously characterized 70 S projection forms, and its three-dimensional localization on the 70 S model established. The release factor-2-binding site was found to be positioned at the ribosomal subunit interface, comprising the stalk-protuberance region of the large subunit and the head-neck region of the concave side of the small subunit.

A novel protease may explain widely differing models for the structure of Artemia salina haemoglobin

Abstract Evidence that a protease is responsible for the proposal of differing models for the structure of the haemoglobin of the brine shrimp Artemia salina is presented. The protease, a minor contaminant in highly purified haemoglobin preparations, is activated by SDS of the SDS-polyacrylamide gel system and readily degrades the haemoglobin subunit. Removal of the protease by chromatography on Con A-Sepharose 4B has shown the haemoglobin ( M r 260 000 ) to comprise two subunits of M r 130 000 .

Interaction of the release factors with the Escherichia coli ribosome: structurally and functionally-important domains

There are two major domains of interaction between the Escherichia coli release factors (RF-1 and RF-2) and each subunit of the ribosome. RF-2 has a binding domain on the shoulder and lower head region of the small subunit at the small lobe distant from the decoding site. This is in close proximity to one of the domains on the large subunit which includes the body dimer of L7/L12 and L11. The other domains of interaction, at the decoding site on the small subunit, and at the peptidyltransferase centre of the large subunit of the ribosome, are some distance from the first two, although the evidence for direct contact with the ribosome is less comprehensive. The release factors may therefore have two distinct structural domains, and in support of this concept RF-1 and RF-2 can both be cleaved into two fragments by papain. Region-specific antibodies, and antibodies against defined peptide within the RF sequences have given an indication that a significant part of an interacting RF molecule is in close proximity to the ribosome surface, confirming an observation by immunoelectron microscopy which suggested that the RF penetrates deeply into the cleft between the two subunits. A region of highly conserved primary sequence between the two release factors from E coli is also conserved in those from B subtilis suggesting it forms an important structural or functional domain. Antibodies against peptides from the N-terminal end of this region strongly inhibit binding of the RF to the ribosome.

Identification of the blue-green chromophore of an abundant biliprotein from the haemolymph of Artemia

Abstract 1. 1. The haemolymph of adult Artemia is frequently coloured blue-green by a glycoprotein complex, artemocyanin I. 2. 2. Artemocyanin II (M r 1.2 × 10 6 ), a structurally modified form of artemocyanin I purified from Artemia homogenates, has a blue chromophore which has been identified as a bile pigment related to biliverdin. 3. 3. It is suggested that the bile pigment is derived from the turnover of haemoglobin and is bound to artemocyanin for transport and excretion.