Nucleic acid synthesis and regulation in blue-green algae.

photosynthetic systems. I t seems as if the blue-green bacteria are a key intermediate in the development of photosynthesis and also in the development of the 2Fe ferredoxins, possibly from ferredoxins containing a 4Fe4S centre. This last point is also interesting in that it has now been shown that the 2Fe-2s centre is formally derivable from the 4Fe4 s centre in analogue compounds that have been synthesized by Holm's group (Mayerle e t al., 1973). There are suggestions that the 4Fe-4S centre is a much more stable complex, whereas the 2Fe2S centre will naturally devolve into a 4Fe4S centre. One can speculate that the 2Fe ferredoxins need to be twice the size of a 4Fe ferredoxin or an 8Fe ferredoxin because it is much more difficult to maintain the 2Fe-2s centre in a given configuration than it is to maintain a 4Fe-4S centre in its normal configuration. This begs the question as to why blue-green bacteria did not use the 4Fe-4S centre; it may have to do with the incorporation of both soluble and membrane-bound ferredoxins into an association with membranes that are required for the photosynthetic systems characteristic of 02-evolving organisms.