Dynamics of co-translational protein targeting

How does sub-cellular localization affect the fate of bacterial mRNA?

Recently a number of seminal studies have revealed that both sequence and spatio-temporal factors govern RNA decay in bacteria, which is crucial for regulation of gene expression. Ribonucleases have been described that not only exhibit sequence preferences, but also are sub-cellularly localised. Furthermore, the RNA itself is distributed in an organised manner and does not diffuse freely or randomly within the bacterial cells. Thus, even within the sub-micrometer distances of the bacterial intra-cellular space, the positions of the enzymes and their substrates are kept in check. Adding to this complexity is the secondary structure and sequence specificity that many, perhaps all, ribonucleases exhibit, including those that are responsible for "general" RNA degradation. In this review, the implications of these novel findings are discussed and specific examples from Staphylococcus aureus are analysed.

The Model [NiFe]-Hydrogenases of Escherichia coli

In Escherichia coli, hydrogen metabolism plays a prominent role in anaerobic physiology. The genome contains the capability to produce and assemble up to four [NiFe]-hydrogenases, each of which are known, or predicted, to contribute to different aspects of cellular metabolism. In recent years, there have been major advances in the understanding of the structure, function, and roles of the E. coli [NiFe]-hydrogenases. The membrane-bound, periplasmically oriented, respiratory Hyd-1 isoenzyme has become one of the most important paradigm systems for understanding an important class of oxygen-tolerant enzymes, as well as providing key information on the mechanism of hydrogen activation per se. The membrane-bound, periplasmically oriented, Hyd-2 isoenzyme has emerged as an unusual, bidirectional redox valve able to link hydrogen oxidation to quinone reduction during anaerobic respiration, or to allow disposal of excess reducing equivalents as hydrogen gas. The membrane-bound, cytoplasmically oriented, Hyd-3 isoenzyme is part of the formate hydrogenlyase complex, which acts to detoxify excess formic acid under anaerobic fermentative conditions and is geared towards hydrogen production under those conditions. Sequence identity between some Hyd-3 subunits and those of the respiratory NADH dehydrogenases has led to hypotheses that the activity of this isoenzyme may be tightly coupled to the formation of transmembrane ion gradients. Finally, the E. coli genome encodes a homologue of Hyd-3, termed Hyd-4, however strong evidence for a physiological role for E. coli Hyd-4 remains elusive. In this review, the versatile hydrogen metabolism of E. coli will be discussed and the roles and potential applications of the spectrum of different types of [NiFe]-hydrogenases available will be explored.