Inter- and intrasyngenic variations of the hemoglobin components in Paramecium caudatum

The presence of chimeric DNA consisting of 5' regions of the hemoglobin and nucleosome assembly protein-1 genes in Paramecium caudatum macronuclear genomic DNA

We detected an unexpected small-sized DNA fragment during polymerase chain reaction (PCR) analysis of the heterogeneity of a macronuclear intergenic region of Paramecium caudatum. Southern blotting of total genomic DNA with the PCR product as a probe indicated that the small-sized DNA fragment constituted part of the macronuclear genome. Sequencing revealed that the PCR product was a chimeric DNA structure that may be generated by tail-to-tail fusion of the 5' region of the hemoglobin (hb) gene to most of the nucleosome assembly protein-1 (nap-1) gene. Short tandem repeats consisting of tetra- and tri-nucleotides exist at the putative cleavage sites in the hb and nap-1 genes, respectively. This feature differs from those found at the boundaries of TA-internal eliminated sequences in the P. aurelia complex and at transposable elements in other species. This suggests that the chimeric DNA is generated by a novel mechanism. Although the chimeric DNA contains the hb and nap-1 promoters, transcripts corresponding to the chimeric DNA were not detected by reverse transcription (RT)-PCR analysis during vegetative cell growth. Possible roles of chimeric DNA are discussed.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Structure of the Paramecium caudatum gene encoding the B-type of the major hemoglobin component

We have found the gene (Hb) encoding the B-type of the major hemoglobin (Hb) component in the ciliated protist, Paramecium caudatum, stock K33 of syngen 3. Gene Hb, which encodes a protein consisting of 116 amino acids (aa), had the same sequence as the A-type Hb in this species, with the exception of 10 nucleotides (nt) reflecting 5 single-aa differences. The coding region was interrupted at the position between codons 62 and 63 by one short intron, which was comparable with the second intron of plant Hb in position, phase and nt sequence. The maximum alignment of the P. caudatum Hb aa sequence with the other globin sequences from prokaryotes and lower eukaryotes indicates that these globins can be classified into two groups: one consists of protist and cyanobacterial globins of about 120 aa, and the other consists of fungal and other bacterial globins of about 150 aa having a significant homology with higher eukaryotic Hbs. These proteins share more than 25% homology with one another in each group. Our finding of the P. caudatum Hb intron corresponding to the second intron of plant Hb suggests that all of the globin-encoding genes have a common origin, and that at least two groups had already existed in the globin-encoding gene before prokaryote-eukaryote radiation.