## 3 Results and Discussion  
With the purpose of determining whether the N terminal part of the protein encodes for a signal peptide or a transmembrane helix, we retrieved all the protein sequences corresponding to the _cytM_ gene that appear in either of the two main repositories of annotated cyanobacterial genes: CYORF and CyanoBase. A total of 18 sequences was obtained (Table 1), and they were then subjected to a structural prediction through Phyre2, which uses the homology recognition (threading) approach. The first striking result was that 15 out of 18 sequences produced almost identical structures with a long, protruding \(\alpha\)-helix (or \"tail\") corresponding to the initial section of the protein (Figure 2A). This was the precise same region whose nature--signal peptide or transmembrane domain--has not been clearly established to this day. In fact, the VLA motif, repeatedly alleged as the cleavage site of the signal peptide, is located exactly at the junction of the tail with the \"body\" of the protein in the newly proposed structure (Figure 2B). The length of this helix motif of 5.1 nm (the average membrane being between 5 and 10 nm wide; Figure 2C) and the remarkably hydrophobic nature of this section (Figure 2D) agree with a membrane anchoring function. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Organism** & **Strain** & **Phyre\({}^{2}\) Structure Matching** & **DB of Retrieval** \\ \hline _Gloebacter violaccus_ & PCC7421 & Yes & CyanoBase \\ _Prochlorococcus marinus_ & MIT9211 & Yes & CyanoBase \\ _Synechococcus elongatus_ & PCC6301 & No * & CyanoBase \& CYORF \\ _Synechococcus_ sp. & WH7803 & No * & CyanoBase \& CYORF \\ _Synechocystis_ sp. & PCC6803 & Yes & CyanoBase \& CYORF \\ _Nostoc_ sp. & PCC7120 & Yes & CyanoBase \& CYORF \\ _Prochlorococcus marinus_ & NAT1A & Yes & CyanoBase \& CYORF \\ _Prochlorococcus marinus_ subsp. _pastoris_ & CCMP1986 & Yes & CyanoBase \& CYORF \\ _Synechococcus elongatus_ & PCC7942 & No ** & CyanoBase \\ _Prochlorococcus marinus_ & MIT9303 & Yes & CyanoBase \& CYORF \\ _Synechococcus_ sp. & RCC307 & Yes & CyanoBase \& CYORF \\ _Synechococcus_ sp. & WH8102 & Yes & CyanoBase \& CYORF \\ _Acaryodchris marina_ & MBIC11017 & Yes & CyanoBase \& CYORF \\ _Cyanodchros_ sp. & ATCC51142 & Yes & CyanoBase \& CYORF \\ _Synechococcus_ sp. & PCC7002 & Yes & CyanoBase \& CYORF \\ _Thermosmechococcus elongatus_ & BP-1 & Yes & CyanoBase \& CYORF \\ _Microcystis aeruginosa_ & NIES-843 & Yes & CyanoBase \& CYORF \\ _Prochlorococcus marinus_ & MIT9313 & Yes & CYORF \\ \hline \end{tabular}  
* The “Phyre\({}^{2\ast}\) column states whether the structure predicted for that specific sequence by the tool Phyre\({}^{2}\) matched most of the structures predicted for the other sequences or was different from the rest.
* First model proposed by [PERSON]\({}^{2}\) did not match the rest, but the second one (the next most probable according to the software’s algorithm) did. *
* None of the models proposed for this sequence by [PERSON]\({}^{2}\) matched the rest.  
\end{table} 
Table 1: Cyt \(c_{\text{M}}\) protein sequences retrieved from CyanoBase and/or CYORF. Figure 2: Visualization of the predicted 3D structures for Cyt \(c_{\text{M}}\). (**A**) shows the superposition of all the 15 predicted structures that coincided, each one displayed in the ribbon representation and bearing a different color. Among all the sequences that gave rise to the same structure, that of _Nostoc_ sp. PCC 7120 was used as consensus model for (**B–D**), since its prediction had both the sequence with the highest coverage (92%) and the structure with the highest confidence index (99.8%). (**B**) displays in red the place where the VLA motif would be located, whereas (**C**) shows the length of the tail in a rainbow-colored model. (**D**) represents the hydrophobicity surface of the model with an orange-hydrophobic/blue-hydrophilic palette. Note the considerably more hydrophobic nature of the tail.