Cylindrospermopsins (CYNs) have been found in species of Nostocales and Oscillatoriales. Among the Nostocales, Rapbidiopsis (Cylindrospermopsis) raciborskii, R. curvata, R. mediterranea, Cbrysosporum (Apbanizomenon) ovalisporum, Cbrysosporum (Anabaena) bergii, Apbanizomenon flosaquae, Apbanizomenon gracile and Anabaena lapponica, have been identified as producers (Hawkins et al., 1985; Banker et al., 1997; Li et al., 2001a; Schembri et al., 2001; Preussel et al., 2006; Spoof et al., 2006; McGregor et al., 2011; Kokocinski et al., 2013). Umezakia natans, a CYN producer from Japan, was originally assigned to the order Stigonematales (Harada et al., 1994), but later genetic analysis suggests that this species belongs to the Nostocales (Niiyama et al., 2011). Cylindrospermopsin producers belonging to the Oscillatoriales are the benthic Microseira (Lyngbya) wollei, benthic Oscillatoria (Seifert et al., 2007; Mazmouz et al., 2010), as well as Hormoscilla pringsbeimii (Bohunicka et al., 2015). Producing and nonproducing strains exist within these species.
The CYN-producing species have different regional distribution (de la Cruz et al., 2013). So far only Rapbidiopsis raciborskii from Australia, New Zealand and Asia have been found to produce CYNs (Saker & Griffiths, 2000; Li et al., 2001b; Wood & Stirling, 2003; Chonudomkul et al., 2004; Nguyen et al., 2017), while none of the R. raciborskii strains from North and South America, Africa as well as from Europe have been found to synthesise CYNs (Bernard et al., 2003; Fastner et al., 2003; Saker et al., 2003; Berger et al., 2006; Yilmaz et al., 2008; Fathalli et al., 2011; Hoff-Risseti et al., 2013). CYN-producing C. ovalisporum have been reported from strains and/or field samples of Australia, Florida, Turkey, Israel and Spain (Banker et al., 1997;
Quesada et al., 2006; Yilmaz et al., 2008; Ak^aalan et al., 2014). In middle and northern Europe, CYN occurrence is largely attributed to the presence of Aphanizomenon sp. and Dolicbospermum spp. (Preussel et al., 2006; Rucker et al., 2007; Blahova et al., 2009; Brient et al., 2009; Kokociriski et al., 2013).
While earlier studies focused primarily on CYN, data on the presence of 7-deoxy-CYN and 7-epi-CYN are increasingly reported. It appears that strains may contain varying shares of CYN, 7-deoxy-CYN and 7-epi-CYN. In strains and blooms of R. raciborskii, as well as strains of Aphanizomenon and Ana. lapponica, ratios of CYN to 7-deoxy-CYN vary between 0.2 and 5 (Spoof et al., 2006; Orr et al., 2010; Preussel et al., 2014; Willis et al., 2015). In Microseira (Lyngbya) wollei and Rapbidiopsis curvata, 7-deoxy- CYN has been predominately found (Li et al., 2001a; Seifert et al., 2007). However, growth conditions may alter the ratio of 7-deoxy-CYN to CYN, most probably due to the fact that 7-deoxy-CYN is a precursor of CYN (Mazmouz et al., 2010). 7-Epi-CYN has been detected in C. ovalisporum as a minor compound, whereas it was up to threefold more abundant than CYN in some Oscillatoria strains (Banker et al., 2000; Mazmouz et al., 2010). No information is available on the distribution and concentration of 7-deoxy-desulpho-cylindrospermopsin and 7-deoxy-desulpho- 12-acetylcylindrospermopsin recently found (in addition to CYN) in a Thai strain of R. raciborskii (Wimmer et al., 2014).
The complete gene cluster (cyr) for the synthesis of CYN was first sequenced from R. raciborskii (Mihali et al., 2008). It spans 43 kb and encodes 15 open reading frames (ORF). The biosynthesis starts with an amidinotransferase and is completed by nonribosomal peptide/polyketide synthetases and tailoring enzymes. Furthermore, the cluster encodes for a putative transporter (cyrK) for the export of CYN from the cells (Mihali et al., 2008). A putative NtcA (global nitrogen regulator) binding site has been identified within the cyr cluster, suggesting that CYN synthesis is influenced by N metabolism (Mazmouz et al., 2011; Stucken et al., 2014).
Since then, homologous clusters or parts of them have been sequenced from further R. raciborskii strains (Stucken et al., 2010; Sinha et al., 2014), C. ovalisporum (aoa, gene cluster; Shalev-Alon et al. (2002), Aphanizomenon sp. (Stiiken &c Jakobsen, 2010), Oscillatoria sp. (Mazmouz et al., 2010) and Rapbidiopsis sp. (Jiang et al., 2014; Pearson et al., 2016). Differences within the gene cluster between strains comprise the order of the cyr genes, flanking genes as well as a sporadic lack of cyrN and cyrO (Jiang et al., 2014; Pearson et al., 2016).