A Cluster of Major Importance in the Proteobacter: ndk through engA

This cluster is almost certainly of major importance. It is spectacularly conserved within the Proteobacteria. In the E.coli cluster, four of the eight genes are essential (see Gerdes, et al).

The genes I think need to be considered are those numbered 1-8:

1: Putative lipoprotein precursor,
2: GTP-binding protein EngA [essential],
3: Putative inner membrane protein,
4: 23S rRNA methyltransferase and Florfenicol/chloramphenicol resistance protein / Radical SAM family enzyme, UPF0063 family
5: Histidyl-tRNA synthetase (EC 6.1.1.21) [essential],
6: Nucleoside diphosphate kinase (EC 2.7.4.6) [essential],
7: 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase (EC 1.17.4.3) [essential], and
8: Putative membrane protein.

There are clearly more interesting genes moving in and out of the cluster (e.g., the gene numbered 9 and annotated as Fimbrial biogenesis and twitching motility protein PilF), but these eight from E.coli K12 (b2511-b2518) are at the center of things.

This level of conservation seems to suggest to me that we are looking at something fundamentally important. I do not understand why it should contain things that are apparently deeply connected in other subsystems (the histidine tRNA synthetase, ndk, 1.17.4.3). However, unraveling this cluster should lead to something important.