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Abstract
Two morphotypes of the cyanobacterial Limnospira indica (formerly Arthrospira sp.) strain PCC 8005, denoted as P2 (straight trichomes) and P6 (helical trichomes), were subjected to chronic gamma radiation from spent nuclear fuel (SNF) rods at a dose rate of ca. 80 Gy·h−1 for one mass doubling period (approximately 3 days) under continuous light with photoautotrophic metabolism fully active. Samples were taken for post-irradiation growth recovery and RNA-Seq transcriptional analysis at time intervals of 15, 40, and 71.5 h corresponding to cumulative doses of ca. 1450, 3200, and 5700 Gy, respectively. Both morphotypes, which were previously reported by us to display different antioxidant capacities and differ at the genomic level in 168 SNPs, 48 indels and 4 large insertions, recovered equally well from 1450 and 3200 Gy. However, while the P2 straight type recovered from 5700 Gy by regaining normal growth within 6 days, the P6 helical type took about 13 days to recover from this dose, indicating differences in their radiation tolerance and response. To investigate these differences, P2 and P6 cells exposed to the intermediate dose of gamma radiation (3200 Gy) were analyzed for differential gene expression by RNA-Seq analysis. Prior to batch normalization, a total of 1553 genes (887 and 666 of P2 and P6, respectively, with 352 genes in common) were selected based on a two-fold change in expression and a false discovery rate FDR smaller or equal to 0.05. About 85% of these 1553 genes encoded products of yet unknown function. Of the 229 remaining genes, 171 had a defined function while 58 genes were transcribed into non-coding RNA including 21 tRNAs (all downregulated). Batch normalization resulted in 660 differentially expressed genes with 98 having a function and 32 encoding RNA. From PCC 8005-P2 and PCC 8005-P6 expression patterns, it emerges that although the cellular routes used by the two substrains to cope with ionizing radiation do overlap to a large extent, both strains displayed a distinct preference of priorities.
Highlights
Licensee MDPI, Basel, Switzerland.Due to the large-scale industrial production of the cyanobacterium Limnospira with its high nutritive value as a feed and food supplement and its use as a major cell factory for a range of biopharmaceuticals and added-value chemical compounds, a thorough understanding of the various genetic and cellular mechanisms in response to variable environmental parameters is important
Seeing stpA being repressed we became interested in this gene because another solute, trehalose, appears to play an important role in the cellular protection of microorganisms against a variety of abiotic stresses including ionizing radiation [85,86] and we thought that perhaps GG synthesis was switched off in favor of trehalose production as we noted in previous irradiation experiments in L. indica PCC 8005
Important is the development of a genetic system allowing site-directed mutational analysis and the isolation and genotypic characterization of naturally occurring ionizing radiation (IR) sensitive L. indica strains—which we have not encountered yet over the past several years of testing isolates from various sources and geographical locations, a variation in IR resistance does exist for Limnospira and Arthrospira strains in the range of 2–5 kGy
Summary
System Alternative (MELiSSA) (https://www.melissafoundation.org/) for efficient O2 production and recycling of CO2 , and the production of biomass as a highly nutritional end product [6] It recently was given the status of type strain to the newly proposed species Limnospira indica [7]. Limospira cells are metabolically active in contrast to previous irradiation experiments We exposed both morphotypes mentioned above (nominated as P6 and P2 subtypes, with respectively helical and straight trichomes) of L. indica PCC 8005 in an attempt to associate the genomic differences between these subtypes with the different metabolic and physiological responses displayed by them when exposed to IR. G tilling microarray (MA) analysis by Roche NimbleGen, USA; h RNA-Seq performed by NXTGNT, Belgium; all transcriptional analyses were based on genome version v5 (ARTHRO_v5) of 15 February 2014, Genbank accession number GCA_000973065 [9]
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