ORIGINAL PAPER Contrasted evolutionary constraints on carbohydrate active enzymes (CAZymes) in selected Frankia strains Arnab Sen . Louis S. Tisa . Maher Gtari . Indrani Sarkar Received: 29 May 2018 / Accepted: 29 September 2018 Ó Springer Nature Switzerland AG 2018 Abstract Carbohydrate active enzymes (CAZymes) are capable of breaking complex polysaccharides into simpler form. In plant-host-associated microorgan- isms CAZymes are known to be involved in plant cell wall degradation. However, the biology and evolution of Frankia CAZymes are largely unknown. In the present study, we took a genomic approach to evaluate the presence and putative roles of CAZymes in Frankia. The CAZymes were found to be potentially highly expressed (PHX) proteins and contained more aromatic amino acids, which increased their biosyn- thetic energy cost. These energy rich amino acids were present in the active sites of CAZymes aiding in their carbohydrate binding capacity. Phylogenetic and evolutionary analyses showed that, in Frankia strains with the capacity to nodulate host plants, CAZymes were evolving slower than the other PHX genes, whereas similar genes from non-nodulating (or inef- fectively nodulating) Frankia strains showed little variation in their evolutionary constraints compared to other PHX genes. Thus, the present study revealed the persistence of a strong purifying selection on CAZymes of Frankia indicating their crucial role. Keywords Carbohydrate active enzymes Á Frankia Á Nodulation Á Codon usage Á Amino acid usage Á Comparative genomics Á Evolution Á Phylogeny Introduction The actinorhizal symbiotic relationship plays a crucial role from an ecological perspective. It consists of an association between members of an actinobacterial genus, Frankia, and several dicotyledonous plant species belonging to eight angiosperm families (Si- monet et al. 1990). This interaction results in the formation of plant root nodules where the symbiotic Frankia reside (Normand et al. 2014) and fix atmo- spheric nitrogen into a biologically accessible form in planta. This source of nitrogen allows plants to survive and grow in nitrogen depleted soil. Furthermore, actinorhizal plants are also able to colonize harsh environments especially in poor and relatively less Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10482-018-1173-y) con- tains supplementary material, which is available to authorized users. A. Sen Á I. Sarkar (&) NBU Bioinformatics Facility, Department of Botany, University of North Bengal, Siliguri 734013, India e-mail: indrani.sarkar89@gmail.com L. S. Tisa Department of Molecular, Cellular and Biomedical Sciences, University of New Hampshire, 46 College Road, Durham NH 03824-2617, USA M. Gtari Institut National des Sciences Applique ´es et de Technologie, Universite ´ Carthage, Centre Urbain Nord, BP 676-1080, Tunis Cedex, Tunisia 123 Antonie van Leeuwenhoek https://doi.org/10.1007/s10482-018-1173-y