Bacteria have developed several defense mechanisms against bacteriophages over 
evolutionary time, but the concept of prokaryotic RNA interference mediated 
defense mechanism against phages and other invading genetic elements has emerged 
only recently. Clustered regularly interspaced short palindromic repeats 
(CRISPR) together with closely associated genes (cas genes) constitute the CASS 
system that is believed to provide a RNAi-like defense mechanism against 
bacteriophages within the host bacterium. However, a CASS mediated RNAi-like 
pathway in enteric pathogens such as Vibrio cholerae O395 or Escherichia coli 
O157 have not been reported yet. This study specifically was designed to 
investigate the possibility and evolutionary origin of CASS mediated RNAi-like 
pathway in the genome of a set of enteric pathogens, especially V. cholerae. The 
results showed that V. cholerae O395 and also other related enteric pathogens 
have the essential CASS components (CRISPR and cas genes) to mediate a RNAi-like 
pathway. The functional domains of a V. cholerae Cas3 protein, which is believed 
to act as a prokaryotic Dicer, was revealed and compared with the domains of 
eukaryotic Dicer proteins. Extensive homology in several functional domains 
provides significant evidence that the Cas3 protein has the essential domains to 
play a vital role in RNAi like pathway in V. cholerae. The secondary structure 
of the pre-siRNA for V. cholerae O395 was determined and its thermodynamic 
stability also reinforced the previous findings and signifies the probability of 
a RNAi-like pathway in V. cholerae O395.