Nucleus A major mechanism of nuclear gene regulation is transcriptional control. Strategies to enhance heterologous gene expression, which occurs at relatively low levels compared to expression in the chloroplast, are the search for more effective promoters (Rasala et al. 2013) and the enhancement of translation efficiency through codon optimisation (Fuhrmann et al. 1999, 2004; Heitzer and Zschoernig 2007; Mayfield and Kindle 1990). The glycosylation patterns of nuclear expressed and secreted proteins from C. reinhardtii remain to be resolved. Studies on the post­translational machinery of this alga would be helpful to exploit this as an option of algal protein production.

Chloroplast Ensuring high heterogeneous gene expression requires the identifica­tion of endogenous transcriptional and translational regulatory elements (Harris et al. 2009; Marin-Navarro et al. 2007; Purton 2007). Although chloroplast gene expression is usually regulated at the translational level (Barnes et al. 2005; Eb — erhard et al. 2002; Nickelsen 2003; Rasala et al. 2010, 2011; Zerges 2000), choice of promoter and 5′ UTRs sequences is of importance due to potential feedback regulations which can interfere with the heterologous protein expression and mRNA stability (Gimpel and Mayfield 2013; Hauser et al. 1996; Manuell et al.

2007) . The 5′ UTRs are believed to regulate ribosome association, transcript sta­bility and the rate of translation (Barnes et al. 2005; Marin-Navarro et al. 2007; Salvador et al. 1993; Zou et al. 2003) while 3′ UTRs influence mRNA stability (Herrin and Nickelsen 2004; Lee et al. 1996; Monde et al. 2000; Stern et al. 1991) or may interact with 5′ UTRs (Katz and Danon 2002).

Mitochondria In photosynthetic organisms, mitochondria are the organelle with the greatest diversity in size and structure, ranging from 15 kb linear DNA mole­cules in Chlamydomonas to 1.0 Mb in angiosperms. To date, Chlamydomonas is the only photosynthetically active organisms for which mitochondrial DNA has been altered, which significantly limits insights into translational and transcriptional control. Attempts have been made with plant mitochondria to utilise in vitro DNA and RNA import, and electroporation of isolated mitochondria has been used to gain further information about transcription and post-transcriptional processing (Remacle et al. 2012).