@article{Reimann_Alkhnbashi_Saunders-Struc_const_and-NAR2017,
author = {Reimann, Viktoria and Alkhnbashi, Omer S. and Saunders, 
          Sita J. and Scholz, Ingeborg and Hein, Stephanie and 
          Backofen, Rolf and Hess, Wolfgang R.},
title = {Structural constraints and enzymatic promiscuity in the 
         {Cas6}-dependent generation of {crRNAs}},
journal = {NAR},
year = {2017},
doi = {10.1093/nar/gkw786},
volume = {45},
user = {sita},
pmid = {27599840},
pages = {915-925},
number = {2},
issn = {1362-4962},
abstract = {A hallmark of defense mechanisms based on clustered 
            regularly interspaced short palindromic repeats (CRISPR) and 
            associated sequences (Cas) are the crRNAs that guide these 
            complexes in the destruction of invading DNA or RNA. Three 
            separate CRISPR-Cas systems exist in the cyanobacterium 
            Synechocystis sp. PCC 6803. Based on genetic and 
            transcriptomic evidence, two associated endoribonucleases, 
            Cas6-1 and Cas6-2a, were postulated to be involved in crRNA 
            maturation from CRISPR1 or CRISPR2, respectively. Here, we 
            report a promiscuity of both enzymes to process in vitro not 
            only their cognate transcripts, but also the respective 
            non-cognate precursors, whereas they are specific in vivo 
            Moreover, while most of the repeats serving as substrates 
            were cleaved in vitro, some were not. RNA structure 
            predictions suggested that the context sequence surrounding 
            a repeat can interfere with its stable folding. Indeed, 
            structure accuracy calculations of the hairpin motifs within 
            the repeat sequences explained the majority of analyzed 
            cleavage reactions, making this a good measure for 
            predicting successful cleavage events. We conclude that the 
            cleavage of CRISPR1 and CRISPR2 repeat instances requires a 
            stable formation of the characteristic hairpin motif, which 
            is similar between the two types of repeats. The influence 
            of surrounding sequences might partially explain variations 
            in crRNA abundances and should be considered when designing 
            artificial CRISPR arrays.}
}