Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder that is characterized by degeneration of the anterior horn cells of the spinal cord, which leads to the axial and limb weakness associated with muscle atrophy. SMA is classified into three groups based on the clinical severity: type I (severe), type II (intermediate) and type III (mild). All three clinical subtypes of SMA are caused by mutations of the SMN1 gene. More than 95 % of SMA patients show homozygous deletion of SMN1. It is thought that SMN2, which is a highly homologous gene of SMN1, compensates for the SMN1 deletion to some degree. To clarify the relationship between SMN2 and the disease severity of SMA, we performed fluorescence-based quantitative polymerase chain reaction assay of the copy number of SMN2 in 27 patients (11 type I and 16 type II-III) homozygous for SMN1 deletion. The SMN2 copy number in type II-III patients was 3.1 +/- 0.3 (mean +/- SD), which is significantly higher than that observed in type I patients, 2.2 +/- 0.6 (P < 0.01). However, three of the 11 type I patients carried 3 SMN2 copies. A type I patient with 3 SMN2 copies was studied further. RT-PCR analysis of the patient showed a trace of full-length SMN2 mRNA species, but a large amount of the truncated SMN2 mRNA species lacking exon 7. In conclusion, SMN2 alleles are not functionally equivalent among SMA patients, although in general the SMN2 copy number is correlated with the severity of SMA. Genetic background influencing splicing mechanisms of the SMN2 gene may be more critical in some SMA patients.

Bradyrhizobium sp. S23321 is an oligotrophic bacterium isolated from paddy field soil. Although S23321 is phylogenetically close to Bradyrhizobium japonicum USDA110, a legume symbiont, it is unable to induce root nodules in siratro, a legume often used for testing Nod factor-dependent nodulation. The genome of S23321 is a single circular chromosome, 7,231,841 bp in length, with an average GC content of 64.3%. The genome contains 6,898 potential protein-encoding genes, one set of rRNA genes, and 45 tRNA genes. Comparison of the genome structure between S23321 and USDA110 showed strong colinearity; however, the symbiosis islands present in USDA110 were absent in S23321, whose genome lacked a chaperonin gene cluster (groELS3) for symbiosis regulation found in USDA110. A comparison of sequences around the tRNA-Val gene strongly suggested that S23321 contains an ancestral-type genome that precedes the acquisition of a symbiosis island by horizontal gene transfer. Although S23321 contains a nif (nitrogen fixation) gene cluster, the organization, homology, and phylogeny of the genes in this cluster were more similar to those of photosynthetic bradyrhizobia ORS278 and BTAi1 than to those on the symbiosis island of USDA110. In addition, we found genes encoding a complete photosynthetic system, many ABC transporters for amino acids and oligopeptides, two types (polar and lateral) of flagella, multiple respiratory chains, and a system for lignin monomer catabolism in the S23321 genome. These features suggest that S23321 is able to adapt to a wide range of environments, probably including low-nutrient conditions, with multiple survival strategies in soil and rhizosphere.

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